If you'd get one in US and jailbreak it, you'd be out of luck if you'd need it serviced. But otherwise - why not. Or get one from Switzerland or Italy or whereever they sell those unlocked. No need for the UK premium chrges.
O2 sucks and I say that (1,90 euros per minute to call US at any hour of day?!!!) but my experiences with their suckiness is with the Irish counterpart.. I hope O2 in UK sucks less. But with all the other providers out there, I'd probably have a look at the others too.
I'd probably go again with Orange as they are usually cheapest. And - depending on how long you plan to stay there, and how you will be set internet wise, maybe have internet where you live (or get mobile broadband - Orange has
that available too, with a bunch of other options). Probably a simple prepay phone and one of those net dongles and use your Mac for calling home? There are plenty of other options to O2 - Orange, 3 (how about a
skypephone?), Vodafone...
The most useful feature in iPhone for calling abroad for what I've tried is Skype. That with Skype out minutes - I've used it for calling to a bunch of landlines at a few cents a minute vs what the international charges are (
always ridiculous). If you'll decide to just get O2 iPhone you probably want to use Skype when you can...
One thing: if you don't really use mobile phone a lot, there is no reason to go with contract. I've been using only pre-pays for nearly 10 years now (and before then contracts).