JAR-FCL commercial licence theory exam questions are drawn from the central JAA question bank. However, JAR-FCL PPL theory questions are not - they are adminstered at national level.
Thus there is no guarantee that a pass in the JAR-FCL PPL theory exams sat in the UK will be acceptable by the Greek authorities - you MUST check with them first or face the prospect of sitting all the exams again.
Even if the Greek authorities will accept a pass in the JAR-FCL PPL theory exams sat in the UK (which, frankly, I doubt), can you honestly imagine any UK RF welcoming you in with open arms to sit the exams, only for you to disappear off to Greece to do your flying? I would unhesitatingly charge the full CAA rate for anyone trying that on!