Is this not an alternate history? Seems to say right in the title that this is an alternate history

I know full well that such an event is impossible, but I want to play it anyways, if only because it is simply alternate history and I am interested in such a scenario.

Edit:

I should Clarify, I don't need to have all of Britain, I'd be fine with a little nation of Slavs or even Romanized/Celticized Slavic peoples