I'm curious as to why there is such a massive disparity between the US prices and UK prices on a BTO Mac Pro - given that they both ship from china?

I've priced up a system and it comes in at over £1000 cheaper if shipped to a US address, that's taking into account UK VAT and import duty, this is what really pisses me off!!! Even when you compare the prices of individual components it appears to be a case of US prices being £100 cheaper here and there.