What would be better -- buy an iPhone 5 in January and take it to the UK with me and use it there? Or rent a phone in the UK and hold off on the iPhone until I start my next semester here at home, next September? If I rent, I will want to rent a smartphone because I do want to be able to use travel and mapping apps while there.
Or a third option, buy the iPhone in the UK. This is unlikely to be the cheapest option based on the cost of the handset itself, but maybe you'll save money by having access to a better plan.