The BBC has an article describing research by Chris Stringer and others on the human occupation of Britain.
Scientists now think there were seven gaps in the occupation story - times when there was probably no human settlement of any kind on these shores. Britain and the British people of today are essentially new arrivals - products only of the last influx 12,000 years.
"Australian aboriginals have been in Australia longer, continuously than the British people have been in Britain. There were probably people in the Americas before 12,000 years ago," Professor Stringer explained.
People probably managed to establish permanent presence in Australia and the Americas because these were attractive, resource-rich areas for human occupation. Britain, on the other hand, was unsuitable for humans, and was colonized in an opportunistic manner whenever climatic conditions allowed it. The foundation and abandonment of the Norse settlements in Greenland and the Americas should probably be thought of as historical analogues to the multiple failed attempt to colonize Britain.