I think 3G coverage has perhaps improved over the last year or so. I do mostly use it in London where I always get 3G and at least the fallback to GPRS is completely transparent (it can happen mid "call" without problems). Elsewhere it is variable, but it's worked in all the cities I've tried.
Web n' Walk is cheap at 20 quid a month but disallows use of VoIP and any kind of instant messenger. Mind you, with an SSH tunnel...