[I started on this work, and then found this webpage, which pretty much did what I was seeking to do, at least for the roman period. It does not contain links, but a quick google will help. It is not absolutely complete, as is claimed, but as near as damn it! So I am abandoning this post in its pressent form. I might as well present this partial (and unnecessary!) project. Enjoy!]
The internet is full of lists of the Celtic, Roman and Early Medieval pagan gods and goddess of the British Isles. Most are well packed with finely written (and not so finely written) stories and interpretations of their meanings.
However I will seek a slightly different path from most.
I like context.
And this means I want sources. I want to know exactly where and when the name re-entered the modern consciousness.
As such I will miss a lot of possible deities who surfaced in the folklore and myths of the last millennium, even if there is good reason to believe they are of more ancient stock. Most notably I will miss out all the stories from both the Welsh and the Irish ancient manuscripts as they all post date my cutoff point of 1000AD. Perhaps this is why I did a degree in Archaeology, and why I am now so interested in reading the old books of the folklore of my area - I love the imagination of the story, but also I like knowing the structure of the narrative, how each piece fits in (or not!). Unfortunately most archaeological finds cannot be dated exactly - if I am able I will include it.
This is by no means a complete picture of modern knowledge of 'ancient' deities (I am using ancient here to mean older than 1000 years), it is simply my journey so far.
I hope it is an interesting starting block for your explorations...
Abandinus - (Inscribed on a bronze feather - possibly a votive offering - reading "DEO ABANDINO VATIAVCVS D S D" - Translated as "To the god Abandinus, Vatiacus dedicates this out of his own funds."*)
Aericurus - (Found at Corbridge, Northumberland. I could not find the exact nature of the evidence, but it is described as being male, naked and depicted with a cup and urn.*)
Adraste - (Mentioned in a text by the Roman Dio Cassius.
Here is one translation of Dio Cassius, writing of Boudicca, queen of the Icini tribe who lived in roughly the area of modern Norfolk -
"When she had finished speaking, she employed a species of divination, letting a hare escape from the fold of her dress; and since it ran on what they considered the auspicious side, the whole multitude shouted with pleasure, and Buduica, raising her hand toward heaven, said: "I thank thee, Andraste, and call upon thee as woman speaking to woman; for I rule over no burden-bearing Egyptians as did Nitocris, nor over trafficking Assyrians as did Semiramis (for we have by now gained thus much learning from the Romans!), much less over the Romans themselves as did Messalina once and afterwards Agrippina and now Nero (who, though in name a man, is in fact a woman, as is proved by his singing, lyre-playing and beautification of his person); nay, those over whom I rule are Britons, men that know not how to till the soil or ply a trade, but are thoroughly versed in the art of war and hold all things in common, even children and wives, so that the latter possess the same valour as the men. As the queen, then, of such men and of such women, I supplicate and pray thee for victory, preservation of life, and liberty against men insolent, unjust, insatiable, impious,— if, indeed, we ought to term those people men who bathe in warm water, eat artificial dainties, drink unmixed wine, anoint themselves with myrrh, sleep on soft couches with boys for bedfellows,— boys past their prime at that,— and are slaves to a lyre-player and a poor one too. Wherefore may this Mistress Domitia-Nero reign no longer over me or over you men; let the wench sing and lord it over Romans, for they surely deserve to be the slaves of such a woman after having submitted to her so long. But for us, Mistress, be thou alone ever our leader.
Having finished an appeal to her people of this general tenor, Buduica led her army against the Romans; for these chanced to be without a leader, inasmuch as Paulinus, their commander, had gone on an expedition to Mona, an island near Britain. This enabled her to sack and plunder two Roman cities, and, as I have said, to wreak indescribable slaughter. Those who were taken captive by the Britons were subjected to every known form of outrage. The worst and most bestial atrocity committed by their captors was the following. They hung up naked the noblest and most distinguished women and then cut off their breasts and sewed them to their mouths, in order to make the victims appear to be eating them; afterwards they impaled the women on sharp skewers run lengthwise through the entire body. All this they did to the accompaniment of sacrifices, banquets, and wanton behaviour, not only in all their other sacred places, but particularly in the grove of Andate [Andraste]. This was their name for Victory, and they regarded her with most exceptional reverence."*)
Apollo Cunomaglos - (Inscriptions in a temple at Nettleton Shrub, Wiltshire*)
Camulos - (Appears in the name "Camulodunum" (Colchester), which itself first appears in the Antonine Itinerary, a roman text.*)
Diana - (Roman inscription on an altar or statue base in Bath translated as "To the most hallowed goddess Diana, Vettius Benignus, a freedman, fulfilled his vow"* Image)
Divinus Claudius - (Claudius was a roman emperor who was raised to the status of divinity around the time of his death. A temple was built in Colchester.* It was mentioned by Tacitus and by Seneca*)
Loucetius Mars - (Loucetius is a celtic word for light, Mars was a roman god, the two have been combined.
From a roman altar in Bath inscribed "Peregrinus, son of Secundus, a citizen of Treves, to Loucetius Mars and Nemetona, willingly and deservedly fulfilled his vow."* Image.)
Lug - (Found in the name Luguvallo - modern Carlisle* - the name appears first in the 2nd century*)
Mars (Marti) - (Curse tablets at a shrine in Uley, Gloucestershire* Inscription and translation found here, where the name could be twinned with Mercury.
Also on a curse tablet from Marlborough Down. Inscription, translation and image found here.)
Mars Medocius - (Dedication on a bronze plaque in Colchester. A transcription and translation are to be found here.)
Mars Nodons, Nudens Mars - (Named on plaque at Lydney, in the Forest of Dean as "M(ars) Nodons.*
Also named on plaque at Lydney, in the Forest of Dean as "Nudens Mars".*
Also probably named on mosaic at Lydney, in the Forest of Dean as "MN".* )
Mars Olloudius - (Inscription on the base of a stature at Custom Scrubs, Gloucestershire read as "Mars Olloudius"* Image)
Mars Silvarnus - (The inscriptions and translations of a curse tablet from Uley, Gloucestershire can be found here.)
Mercury (Mercurii, Mercurio, Mercurium) - (Plaques, altars and curse tablets at a shrine in Uley, Gloucestershire*. The inscriptions and translations of curse tablet from Uley, Gloucestershire can be found here, here, here, here, here, here, here, here, here, The name on one occasion could have been combined with Mars.
Another curse tablet invoking Mercury, of unknown providence, was bought in Bristol in the 1980's. It was probably from the surrounding area. A transcription, translation and image is to be found here.)
Minerva - (Found on a stone temple dedication plaque from Chichester. Transcription, translation and image are here.)
Nemesis - (Name on a curse tablet found at Caerleon*. A transcription, translation and image may be found here.
Also a dedication in Chester*)
Nemetona - (Nemet is the Celtic word for groves, probably sacred groves.
From a roman altar in Bath inscription, translated as "Peregrinus, son of Secundus, a citizen of Treves, to Loucetius Mars and Nemetona, willingly and deservedly fulfilled his vow."* Image.)
Neptune - (Found on a curse tablet recovered from the river Hamble*. Transcription, translation and image are here.
Also on a curse tablet recovered from the river Thames at London*
Also on a curse tablet recovered from the river Oose at Brandon, Norfolk*
Found on a stone temple dedication plaque from Chichester. Transcription, translation and image are here.)
Numen - (Dedication to the spirit of a living emperor - semi-deified - found in London on a fragmintary inscription now lost*
Also on a stone dedication for part of a theater at Bourgh-on-Humber*. Transcription and translation found here.)
Numinibus Augusti - (Venerating a living emperor as a god was frouned on in the Roman world, but not venerating the spirit of his position (here it is the Emperor Augustus). Found on a stone dedication at Colchester. A transcription and translation are found here.)
Niski - (Found on a curse tablet recovered from the river Hamble. Transcription, translation and image are here.)
Nodens - (Named on a curse tablet at Lydney, in the Forest of Dean*. Transcription, translation and image are here.
Also found on two statues from Lancaster.* )
Romulus - (Statue from Custom Scrubs, Gloucestershire, inscribed on the back - "DEO ROM[V]LO GVLIOEPIVS DONAVIT IVVENTINVS FECIT" ~ Translated as "For the god Romulus, Gulioepius has offered this, Juventinus has made it."* Image)
Senuna - (Probable votive plaque found by a mettle detectorist near Baldock in Hertfordshire - D SENVNE FLAVIA CVNORIS V S L M Translated as “To the goddess Senuna, Flavia Cunoris paid her vow, willingly, deservedly” With other possibly connected objects*)
Silvanus (Silvano) - (Curse tablets at a shrine in Uley, Gloucestershire* The inscriptions and translations can be found here.
Also found on bronze plaque at Colchester. Transcription and translation found here)
Silvanus Callirius (Silvano Callirio) - (Silvan was a roman god, Callirius was unique to Britain. The combination was found on bronze plaque at Colchester. Transcription and translation found here))
Suli[s] Minerva - (Minerva is a Roman goddess, Sulis was probably a Celtic deity (male or female), their names joined.
On an altar at Bath - "DEAE SVLIMI NERVAE SVLINVS MATV RIFIL VSLM"*. [DEAE SVLI MINERVAE SVLINVS MATVRI FIL VSLM]- Translation ~ "To the goddess Sul Minerva, Sulinus son of Maturus willingly and deservedly fulfilled his vow."*)
Suli[s] - (Also on an altar in Bath an Inscription translated as "To the goddess Sulis of the welfare and safety of Marcus Aufidius Maximus, centurion of the Sixth Legion Victrix, Aufidius Eutuches, his freedman, willingly and deservedly fulfilled his vow."*
Almost identically was a second altar at Bath translated as "To the goddess Sulis for the welfare and safety of Aufidius Maximus, centurion of the Sixth Legion Victrix, Marcus Aufidius Lemnus, his freedman, willingly and deservedly fulfilled his vow."*
Inscribed on a statue base at Bath - "DAE SULI L.MARCIVS MEMOR HARVSP D.D". Translation ~ "To the Goddess Sulis L[ucius] Marcius Memor, Haruspex, gave this gift"* Image
On a grave stone is the inscription translated as "To the spirits of the departed, Gaius Calpurnius Receptus, priest of the goddess Sulis, lived 75 years; Calpurnia Trifosa, his freedwoman and wife, had this set up."*)
The Sulevis Mothers - (Inscription on a stone altar dedication at Colchester. A transcription and translation are to be found here.)
The Suleviae - (Roman inscription on a statue base in Bath, translated as "To the Suleviae, Sulinus, a sculptor, son of Brucetus, gladly and deservedly made this offering". Image A similar inscription was made at Gloucestershire* )