The 1st Crusaders, late 11th century, brought back documents to Europe that undermined the official story of Jesus and his followers. They couldn't publish it for fear of reprisals from the Catholic Church so they had it rewritten in the form of the King Arthur romances. These stories were set in Britain because that's where Jesus was imprisoned for twenty years and died. Hence the hymn "Jerusalem" - "and did those feet, in ancient times, walk upon England's pastures green, etc, etc."
The real Jesus was a warrior king who took on Rome for control of Judea and lost. He was crucified but taken down from the cross while still alive (Josephus in his book "Life" says he was the one who got him reprieved) Jesus was then exiled in a Roman fortress in Britain to prevent his family from starting up another rebellion in revenge. Christianity was then invented claiming that the Messiah had indeed come, but was different from what the Jews had expected. This was all politics.
If you think this is fanciful, you should know that this sort of religious merging is exactly what the Romans did with all countries they conquered. That was the beauty of Paganism, it allowed you to easily merge one religion with another in order to keep the peace - the Pax Romana. Christianity does just that, and is clearly a Roman invention.