I have been using Vonage for many years. They supply a box that plugs into your broadband router at one end and a standard UK landline phone at the other. The service has been excellent. In our case they do need to know your geographical point but they can offer you a dialing code that is elsewhere in the UK, ie as I was using for business they offered me a London number to add a bit of prestige to things. We are Reading based and use 0118 dial code. Local outgoing calls don't need the dial code. When we are at another location in North Yorkshire I take the box with us and thus I have my normal number. I just need to remember to add dialing codes to any numbers that are not in Reading. It does confuse the folk you ring though, you seem to be calling from Reading and then you say see you in a few minutes and they expect you to take 5 hours to drive there and you walk through the door after 10 mins...........
Vonage probably aren't the cheapest judging by the other suggestions..... I think I am paying 10 ish UKP per month but it is a business expense............
Rans6...............