I know this is an older post, but... Callcentric.com will give you a VOIP phone number for about $5 per month in the USA ($2.95 for the incoming number, and $1.50 for the 911 emergency listing, plus taxes). Incoming calls are free, outgoing calls are billed at an extremely reasonable amount. Or they've got more expensive plans, with varying amounts of minutes, or unlimited minutes. I don't use the phone here at the house often, but keep it for emergencies.
The last time I was traveling in the UK (I'm still in the States now) I forwarded my Callcentric.com number to my UK cell phone. Worked fine. Alternately, you can have a phone directly hooked up into your router via a VOIP adapter and pick up the USA calls via a handset if you are in the UK.
The only downside is you have to be a bit of a techhead to get it all set up. Took me a few hours and a lot of emails back and forth (there is no telephone helpline) to get it set up. Once it was up, it's been running just fine. Of course, if your internet goes out, so does your phone. But that happens here in the states if you have phone through your cable company anyway and the cable goes out.
I think there's also a thing called softphone software that CC uses that allows you do do the dialing on the internet, etc., and talk through your computer - no need for an actual phone? It's all on their website, so it may be of interest.
If we are allowed to come to the UK to stay, I'll be taking my setup with me and all the incoming calls are still free from the States and treated as if the USA caller had dialed my USA number (no overseas charges for them), and outgoing are still pennies a call, so if I need to wrap up ends there I'll be able to do it. They also have UK phone numbers now, too, but they cost a bit more per month, I think. Still under $20, if memory serves. (edit - it appears to be $5.95 per month for the incoming line, and you choose the outgoing call plan.)
You might check them out. We've been really pleased with them. Used them for several years now with no problems at all.