Alternately, you can buy the data SIM that Matan mentioned and some skype in credit so that your callers can call a UK number and you don't pay expensive international call rates. Otherwise if you can live with outgoing calls only (don't pick up when thry call and call them back later as you will see who is calling you), you can simply open a skype out or SIP account. Most cafes and hotels have free wifi so you can use that to make cheap calls and won't need any new sim.