Interface is a bit clunky, but this site can help locate mobile masts and give iPhone users some idea of what kind of coverage they can expect. For example, I get Edge at home (I'm fairly close to an O2 mast), but not at the office (where I am more than twice as far away from an O2 mast - but I still get a workable signal for calls, texts, and e-mail)
http://www.sitefinder.ofcom.org.uk/
http://www.sitefinder.ofcom.org.uk/