Unfortunately Truphone do not solve the problem. But thanks anyway El_condor.
I found this forum thread with people trying to do exactly the same as me:
International sim with USA number - PrePaidGSM.net Forum
And they found a solution: Skype.
(US$0,017/min)
But unfortunately Skype puts a big delay to start ringing. Besides that it is not SIP compliant so I can not use sipbroker and Tpad with it.
Edit: A few minutes later....Skype definitely is not a solution anymore. Their new rate to UK +447624 is US$0,25/min.