I quote:
The first morphological evidence came to light in 2007 from the bathymetric maps of the English Channel. Dr Sanjeev Gupta at Imperial College London observed that the existence of long grooves of erosion and deep valleys running longitudinally along the bedrock floor fitted with an extraordinary conclusion. It seems that the freshwater lake filled until, like an overflowing bath, it breached the Dover Strait. A catastrophic discharge of water surged at least once and probably twice down the basin between Britain and France, overwhelming the rivers and streams below it, and spreading out across the basin as a megaflood. This massive southwards discharge of meltwaters merged with the river-water from the Seine, Somme and others, to form the ‘Fleuve Manche’ (Channel River) palaeoriver, one of the largest river systems on the European continent.
Further ice ages followed. Each time, as the glaciers receded, sea levels rose and the Channel would continue to be carved out; as glaciers returned, sea levels dropped and the landmass would once more connect Britain to the Continent. A second megaflood seems to have happened 160,000 years ago, only this time the gap at the Dover Strait was enlarged enough that it would never reform: Britain was now an island.