ITunes abroad? Will foreign gift cards work?

I live in the uk and I'm soon going to get a iPhone 5, but in a few years I may live abroad, I'm not sure if I could still use the phone properly, how would you change the Siri, is it possible to change the country in the settings but keep the language in English so I would still understand the home screen, can you still download English movies? If I somehow changed the country in settings would iTunes then allow me to use a gift card which is from the different country.

For the iTunes store you can only access the content for the country where the account is registered and where you intend to play it back, so for instance if you have a UK address you can only access the UK store.same across the world this is to make the copyright holders happy, if you have an address you can use for billing in the uK you may be able to use it. As for the language, you can set that to whatever is available so yes uk voice can be selected no matter where you are on the planet.
iTunes is very restricted and actively enforces restrictions, if you tell apple the country you want to go to they should be able to advise content availability, you may even loose some of the already purchased DRM content.