I seem to remember a statistic where UK is reputed to have a greater number of dialects than any other country in the world, which probably has a lot to do with the difficulty people have with understanding some accents (tried Glaswegian, anyone?)

Also, IIRC, the colonies tended to have a commonality in heritage of the colonists, too. E.g. at one time, Australia had a "large number" of criminals, there are quite a lot of Dutch in NZ -- there are large parts of NZ with very strong Scots heritage -- New York is famous for Irish heritage, other parts of USA are famous for German, etc. The UK hasn't really had the same homogenisation outside the big cities.