There have been a number of attempts by humans or ancestors to colonize the British Isles, but so far each one has terminated with the onset of the next ice age.
They yo-yo’ed from the currently submerged continental shelf to the current mainland and back, multiple times. The Welsh folk epic refers to the Sinking Lands between Britain and Ireland, and there’s a smaller-scale version of that regarding the Lost Land of Lyonnesse off Cornwall. In the Solent there’s the tales of towns that slipped into the sea during the Middle Ages. Not sure about Lyonnesse, but the Sinking Lands really happened, there’s submerged forests off the coast, and the medieval town traces are still there under the shallowest waters of the North Sea.