You might want to read up on Doggerland: https://www.nationalgeographic.org/maps/doggerland/ https://www.nationalgeographic.org/maps/doggerland/ The TLDR is th

You might want to read up on Doggerland: https://www.nationalgeographic.org/maps/doggerland/ https://www.nationalgeographic.org/maps/doggerland/ The TLDR is that a huge low-lying basin of fertile land around what is now the British Isles was inundated at the end of the last interglacial. (What is now the North Sea/English Channel was one of those river valleys you alude to.) The British Isles today would have been the relatively cold, arid, mountainous uplands survivors migrated into when their fertile, low-lying territory got flooded out. Assuming population density in the chilly uplands (glaciers, remember!) was low, then you'd expect the frequency of surviving stone tools to be lower, reflecting that. People don't leave stuff lying around in places where people don't live!