http://www.whyprophets.com/prophets/celtic.htm

it says:
The First Missionaries:
The sixth century British historian Gildas said the gospel came to Britain at a very early stage – in the last year of Tiberias, AD 37.

http://www.cushnieent.force9.co.uk/advent.html

http://www.britannia.com/church/bond1.html

http://www.write-on.co.uk/history/early_british_christianity.htm

I haven;t read them yet, I just Googled them, and will read and review them.

But it seems the real date of the advent of chriatnity perhaps cannot be clearly definied.