I setup a friends eBay sourced iPhone at the weekend. Like many other UK users, he wanted an iPhone but was already in a contract with another provider (Orange in his case). So here is some useful information we found which may help you of you are in the same position...

The other info you'll need is the GPRS settings for your UK provider - Orange, T-Mobile, Vodafone or O2 (the iPhone doesn't work with 3). You'll need three things, the APN, username and password. For a list of these setting visit filesaveas.com/gprs.html

One last thing. Talk to your provider about adding a data bundle to your tariff. Some operators still charge £2.50 or more per megabyte and if you start using your iPhone browser, email or maps etc you could be in for a nasty surprise!