The main reason for not wanting to be stuck with a US English dictionary is that there's a lot of words that have different meanings in the US and the UK. Obvious things like football (two pretty different sports), pants (underwear or trousers?), is it a biscuit or a cookie, a vest or an undershirt, it's currently Autumn or is it Fall?
http://en.wikipedia.org/wiki/List_of...h:_M%E2%80%93Z