Forget the ASB aspects for just a minute. Wheras William in 1066 would have certainly invaded across it, at some point in the centuries after that, the British would have dug a canal across it, partly for defensive purposes, and partly to allow access from the North Sea to the Channel. This would certainly be pre-Hitler, probably pre-Napoleon. Of course, as an earlier poster said, with this in place, a lot gets butterflied away, so maybe you get rid of Hitler, Nappy and William the Bastard.
Wheras William in 1066...
You're examining a prehistoric geological POD and you actually believe William the Conqueror will still show up? Let alone Hitler?
Do you even understand how this stuff works?