>>>how different are the British and North American versions<<<

They're different enough that the title of the first book is different. The US version is called The Sorceror's Stone, the British The Philosopher's Stone.

One thing in particular that I want to know about is the use of units. Harry frequently states that they must be "miles under the school" in hidden caves and tunnels. Kilometers seems like a much more reasonable statement than miles. I'm curious what the original text says. There were other things that just struck me as a bit odd which made me wonder what was actually written. (like, did they change football to soccer?)

see also:
http://wordsmith.org/board/showflat.pl?Cat=&Board=words&Number=2548&page=&view=&sb=&vc=1