Re catastrophic flooding: I have in front of me a copy of the April 2017 issue of A&G (News and Reviews in Astronomy and Geophysics), open at an article titled "A megaflood in the English Channel". I can't paraphrase five pages of densely-argued prose here, still less include the illustrations; but the subheading is:

In the 2016 Harold Jeffrey Lecture, Jenny Collier describes the discovery of plunge pools and streamlined islands in the English Channel, the geological consequences of a Pleistocene Brexit.

Doggerland is a different case, and somewhat more recent. This Wikipedia article gives a decent summary of all this, both the gradual and the catastrophic. Hope this helps.