Below is a cut and paste from their site as they explain it better than I.
Welcome to the British Surnames website. Here, you can find information on similar surnames, most common surnames, surname distribution maps as well as surname meanings and etymologies
As the name suggests, this site primarily lists surnames commonly found in Britain, but many of these are found in other parts of the world as well - so even if you're not British, your name could well be in here!
I find it a very interesting site.
Bye for now,