Your contract isn't with Apple. It will be with O2, etc, signed up when you get home and register your iPhone, not when you buy it at the Apple store. You might just as well order from your provider
Missed an important point in the last post. You will be getting an unlocked phone at full price from the Apple store (£429 for an 8GB iPhone 4). You would be better getting a subsidised phone from a provider like O2. iPhone 4S has just been announced and will be available later this month