I don't have a US account, so US promo codes wont work for me, and I assume it is the same if a US user tries and uses a UK code. Is there anyway that in the title of threads the dev can put in brackets the region the codes are for? For example: New shooty tilty kill simulator promo codes - [UK] (or [USA]/[JPN]/[AUS] etc)
Promo codes only work in the US. There is no such thing as a UK Promo code or any other country. You need to make a fake US account to activate promo codes.
It's extremely easy to create a US account just for promo codes and freebies, and I know quite a few non-US forum members have secondary iTunes accounts just for this. It is a little weird that this restriction exists, but, whatev.