Abe, maybe it's not Doggerland area but the SOUTH of England he's speaking of that was separated in this poem.
Cornwall could be associated with being Amphitrites throne easy enough.
You mean the Channel area. But Dutch fishermen have dragged up bones and artifacts from the bottom of the North Sea for ages, so many will have assumed the North Sea was once dry land. And much of the Channel had already been formed long before the tsunami caused by the Storegga Slide, but Britain was even then still connected to the European mainland.
And although they didn't have sonar scan or things like that, they were able to measure the depth of the North Sea