As you will know from your Asterix, Britain did not always have gods made in the Meditteranean.
Boudica, the great Queen who led the rebellion against Rome, is said to have evoked the Icenian goddess of war, Andrasta, before battle. She probably did not eat Hare, Chicken or Goose, because they were all sacred animals.
She might even have been a druid herself.
Anyway, we don’t really know.

This is a brief overview of the journey that Britain took (i’m using that in a pretty broad, sort of Roman sense, rather than in sense of a modern political union) to getting one true God the Father Almighty, the maker of Heaven and Earth, oh, and also of course, Jesus Christ his only son.

A Timeline

Year: O*
Year: 43
Claudius finally managed to invade Britain. The Romans created syncretised gods (a bit of Roman god mixed with a bit of Britain god) like Sulis-Minerva. Christianity at this point was just one of many smaller sects and religions within the Roman Empire. A few Christians might have come over as slaves, or soldiers. But it was very early days. The gospels weren’t even written yet. And Christians were not popular in Rome, except when they were being thrown to the lions for entertainment.
Year: 50
At the Council of Jerusalem, the Christians decided that you could eat sausage rolls and still be Christian. This is almost literally the reason that Christianity was able to spread so quickly and so widely.
Year: 60
Massacre of Ynys Môn. The Druids had pulled back to the Island of Anglesey. Suetonius Paulinus wanted to snuff them out, so he took all his Roman forces, perhaps twenty thousand men, and rode across the water. They massacred the Druid families and destroyed the sacred groves. After this, the Druids mostly disappeared off record.
Year: then - 300 ish
Christianity was growing in popularity, but was still not tolerated by the Romans. Three Christian martyrs are recorded in Britain under Roman occupation - St. Aaron, St. Alban & St. Julius. Still this is only a small percentage of those killed across the Empire.
Year: 303
The Diocletianic Persecutions began across the Empire. Also known as the 'Great Persecution'.
Year: 313
The Emperor Constantine converted to Christianity. A turn up for the books. Christianity was now legalised throughout the Roman Empire.
Year: 313-380
Christianity was becoming more popular in Roman Britain. Alongside Celtic and Roman gods.
Year: 380
Christianity became the official religion of Roman empire! So it was technically the official religion of Britain. I think. But a few years later -
Year: 410
The Fall of Rome. Romans withdrew from Britain, but bits of their religions remained, including Christianity, particularly around Wales.
Year: 410-597
In the gap left by Rome, the Anglo-Saxons started arriving. They brought lots of great stories and also their own Pantheon of gods, known for liking specific days of the week: Tiw, Woden, Thor, Frige. So, Britain in these years was a sort of mix, a mess even, of Anglo-Saxon deities, bits of Roman deities, hangovers of druidic memories and Celtic syncretic gods, a world where trees and rivers and running hares were still imagined to have spiritual power.
NOTE: One day of the week, Saturnday, is reserved for a favourite Roman god, who basically gave Britain the festival of Christmas, a great favourite in these islands. Then there are two more days - one for the Sun and one for the Moon. By the time that God the Father arrived, all the days of the week were sadly already taken. God said he just wanted one day of the week. Traditionally, Christians observed this on Friday to Saturday. Then they changed it to Sunday.
Year: 597
Pope Gregory (known for his chanting) wanted to convert the Britons to Christianity. He sent a particularly effective monk, Augustine, with a convoy. Augustine landed in Thanet, and sent a messenger to King Æthelberht of Kent inviting him to meet. The King agreed, on condition it was in the open air, to prevent any Christian magic trickery. Berte, The King's Frankish wife, was already Christian, and she came along too. Long story short, Æthelberht converted and appointed Augustine the first Bishop of Canterbury, laying the foundation of Canterbury Cathedral. More baptisms quickly followed.
Year: 616 - 660
Ups and downs. Æthelberht’s son didn’t like the new religion. But then it did well again.
Year: 660
By 660, there were communities of Christians in almost every British kingdom.
Year: 664
The sixth Bishop of Canterbury was appointed - for the first time, he was a native-born Briton.
Year: 669
Pope Vitalian sent two of his friends - Theodore (of Tarsus, modern day Turkey) & Hadrian (of North African origin) to establish order in what was still a pretty chaotic religious landscape. There were lots of different styles of Christianity. The two of them travelled around the island and were largely unimpressed. Theodore appointed himself the first 'Archbishop' in England.
Year: 679
Theodore called a meeting to establish Roman Christianity as the 'true orthodox faith' of England. The first formal statement on an English Church.
Year: 686
King Arwald of the Isle of Wight - the last pagan Anglo-Saxon king and the last king of the Wihtwara - was killed by Cædwalla of Wessex. Some say this date marks the Christianisation of England. Which is not to say that all the pagan peoples of England totally got the memo.
Year: 723
An English monk, Saint Boniface, travelled to Geismar in Lower Saxony. He found a sacred Oak, Donar’s Oak, and cut it down in front of pagan worshippers. Just like the Romans cut the ancient Oaks of Britain. And so the world turns.
Year: 750
Blathmac, an Irish monk, wrote a poem about Jesus, telling us that he was "better than a prophet, more knowledgeable than every druid’. People seemed to be welcoming Jesus into their lives.
Year: 793
A shock attack by the Vikings upon the famous monastery at Lindisfarne, off the coast of Scotland. The Viking marauders drowned the monks in the North Sea and took some as slaves, and they desecrated the shrines and stole theirs treasures. This shocked Christians throughout Britain and even Europe. And it marked the beginning of the Viking age. This terror drew the various strands of Christian identity closer together.
Year: 1066
Norman invasion. The Normans brought with a more intense relationship with Roman Catholicism. The Pope had blessed the invasion, because William promised to iron out the 'irregularities' of the Anglo-Saxon church and bring it in line with European practice.
Year: 1075
Within a decade of the ‘Norman yoke’, every English-born Bishop has been replaced with a Norman bishop. Although the ‘Norman yoke’ is a concept applied by Victorian historians, the Normans did definitely take control.
Year: 1534
Henry VIII breaks from Rome. The Church of England is established.

After this, it is back and forth and back and forth, and lots of persecutions, and public executions, burnings at stakes, and priests hiding in holes in manor houses and learning to lie, I mean equivocate, I mean lie - and Fox’s nineteen Protestant martyrs were martyred and they are still honoured with nineteen crosses every year in Lewes, on the day that marks the deliverance from Catholic gunpowder terrorist plot. That plot was maybe even a set up. In any case, Catholics could in no way live freely in the land in 1605. There was a law in place even until 2013, forbidding the monarch of England from taking a Catholic to spouse. Which is a hangover from the Civil War because Charles I, who was beheaded on Whitehall, had a Catholic wife. Never again. And then there is the asylum given to the Huguenots escaping the extensive massacres of St Bartholomew, who built such nice houses in the East End of London. And most memorable of all, the brief period when we became a Christian theocracy, and Christmas was cancelled. Something all Britains could agree was zero fun, which led to the Plum Pudding riots.
And all that.

But the many many terrible struggles are too much to go into detail here.
And are not currently what I’m thinking about.
I’m just thinking about how we ended up with this religion in the first place.
Anyway, if anyone wants to tell me anything about this, do let me know.

* Year O is my favourite year. But in reality, there is no year O. It is just an imagined moment in time. We go straight from -1 BCE to 1 CE. CE used to be AD, Anno Domini (in the year of the Lord). This system was put in place in the sixth century. We now call it BCE and CE, (Before) Common Era. This is meant to include people like me, for whom Christ is not my Lord. Some people get annoyed at this kind of thing.