Once again, it will depend upon distribution rights (and the way that iTunes configure their software).
As an analogy, consider the way that the BBC make their local radio programs available online. I can listen to, say, BBC Radio Suffolk, anywhere in the UK or throughout the world. However, as soon as they transmit a commentary on an Ipswich Town football match, listeners outside of the UK will find they can no longer listen to the station. (This is because the BBC don't have international broadcast rights for football commentaries).
The BBC have got their software well-configured so that it
only blocks 'illegal' transmissions but allows everything else through. It's possible that iTunes might have their software configured to block
all downloads to the UK from their US site, irrespective of whether they hold international distribution rights. It's difficult to be sure but I certainly wouldn't invest any money on an iTunes gift card without first e-mailing the iTunes people to check on the availability of downloads to the UK:
http://www.apple.com/support/itunes/store/down load/#form
Chris