Current Archeology has recently had some cool articles about the earliest humans in Britain. Settlement has been interrupted a number of times with ice ages. Humans have moved to Britain during warm periods (Britain was warm enough for elephants at one point!), and been driven away by ice ages. Of course each of these movements are independent, since the new settlers would of course have no idea of earlier occupations.
http://www.archaeology.co.uk/articles/the-ebbsfleet-elephant.htm
As the local climate oscillated between polar desert conditions and temperatures akin to the modern Mediterranean, humans were able to gain temporary footholds before being swept away by successive ice ages. This process was repeated at least eight or nine times, but finally, as the last ice-sheet receded c.12,500 years ago, a new wave of migrants recolonised Britain, and this time they were able to cling on.
hi.. there lots of room for more input (and more links!), but check out this post from yesterday for a few useful links on pre-Celtic Britons
What language was spoken in England before the Anglo-Saxon invasion?