Format of number matters
I just tried sending myself an email with European numbers, and the format matters. Key: if you're address book is setup for US numbers, make sure the country code is set off. So, for England, make sure the "44" is set off from the rest of the number. What comes after doesn't much matter, but iCal will see the 44 as a dialing code if it is set off. If it isn't, it just sees it at as a bunch of numbers. Also, you can either include the "+" symbol, or not, makes no difference. So, make sure the numbers for England are formatted as follows:
+44 (0) 871 224 1111
44 (0) 871 224 1111
+44 08712241111
44 08712241111
(Telephone number for Aston Villa FC online store, fyi)