We went to London last year and rented UK capable cell phones through our current cellphone company (verizon) before we left. Verizon shipped the phones to us before we left, which included instructions on forwarding calls to our regular number to the UK phone.
Outgoing calls weren't too expensive, but the best part was that incoming calls were free! So if we needed to call home, we'd call and then have the person just call us back (they would only have to dial our local number).
It was definitely worth it, and was nice to have a cell phone while we were there for making calls within the UK as well.