if you have a contract mobile call 07744 200 007 and use that for calling thai mobiles. I use it on tmobile- it only costs you free minutes, or if you have used them all the price of x-network minutes.
Tmobile have, however, text me today saying they are going to charge 12p/min for calls to 07744 numbers from 1st June
No idea how much it would cost from the virgin landline though