During the last ice age, Britain essentially didn't exist. It was just a highland area in northern Europe. The English Channel was a broad river valley that contained the Seine River, and to the east there was once a massive river where the Thames, the Rhine, the Elbe, and many of northern Europes lesser rivers combined into one. This huge river would have been several miles wide by the time it was done snaking across the fairly flat plain, and would have easily been one of the largest rivers on the planet.