This isn't exactly the same idea but it could be but there is more rattling around in here so:
The Blitz here manages to qualify as Britain's Darkest Hour, thus triggering the return of Arthur from the Realm Avalon.
He does not speak a lick of modern English. He speaks an unholy mishmash of Brittonic and Late Classical Latin.
(Honestly I can see the latter becoming a plot point if they manage to get their hands on a Roman Catholic priest to act as a translator. It wouldn't be a perfect arrangement, but probably better than anything else.)
Truthfully he probably gets mistaken for a madman.
Somehow manages to steal a Spitfire out from under the RAF's nose, proceeds to use it to bring down like half an enemy squadron on his own, then lands in a field in the middle of nowhere.
Police and RAF converge on his location on account of the whole "stealing a plane" thing. They eventually overwhelm him with sheer numbers, but he manages to knock out an impressive number of them in the process. I mean, come on. It's Arthur.