I tried Sipgate previously - their service was pretty good but then I started getting steadily poorer call quality which I could not resolve despite their tech support's best efforts. I did hear that there was something between Sipgate and Telewest/Virgin Media that didn't chime, but I couldn't get to the bottom of it. They do have a very nice subscriber's website: you can compile phone books, send SMS and monitor your in/out calls.
But forced as I was to move I ended up with Draytek (www.draytek.org), who I think are the cheapest (on their silver tariff). Technically they have been very good, my call quality has really never faltered, and their tech support (via email) has always been quick and efficient. However, compared to Sipgate their's is a pretty "bare bones" service - you can see and download your call (out only) history, but that is about it for frills and their website was put together by a trainee. Oh, but they do offer a fax-to-email number, and free voicemail. There is no charge for a local landline number, no subscription fee - just a very cheap call rate. In the end I am glad I moved to Draytel, I still can't believe I will only spend about £45 for all the calls I make in a year and get all the things my regular teleco likes to charge extra for (itemised billing, caller ID) for free.