The price on the US Apple website is exclusive of tax. Most people in the US will then have to pay a sales tax of 6-9% on top of that depending on what state they're in (makes our 20% VAT look like a joke!).
That still doesn't account for the full price difference. I guess shipping from china is cheaper also because they just have to cross the Pacific, but to get it to the UK they have to travel around Africa and past the dangerous area's like Somalia.