Can I buy an iPhone4 in America and use it with my Orange SIM card in the UK?

Orange (the blighters) won't let me upgrade my phone - which is rubbish. To switch to an iPhone 4 will cost �600. Therefore I was going to ask a friend in the US to buy me one (as they are supposedly �200 over there compared to �500, for a 32GB, over here) and send it to me. I was then going to change my Orange tariff to an iPhone tariff and use my exisiting SIM in the new iPhone. Will this work?