One thing to take note of is that the warranty for the iPhone is based in the original country of purchase. If you purchase them in the US then if you have any warranty related issues you will need to ship the phone(s) back to the US to someone you know to take into Apple for replacement. With shipping and insurance this could end up costing you more. An Apple store in the UK will help you up to the point where the phone needs to be replaced. They MAY help but are not required to.
One thing to keep in mind.
Good luck.
I think you'll find that Apples warranty conditions are worldwide. We've had iPads, the occasional iPhone and plenty of MacBooks come through our repair centre, from all over the world, and Apple has honoured all the warranty claims.
So far, we've never had to turn anyone away.