[tarmacscratcher]

Is the phone already sim free? i.e. not tied to a U.K. network
If it is then it needs to unlocked by connecting it to a pc with the relavant software, any good mobile phone dealer can do this.
Then normally it's just a matter of putting the greek sim in then. I've got Dutch and Russian sim cards,don't have to pay the extortionate British network costs then.