I believe there is a CAA approved school in Spain
That would be a UK registered facility, yes, that's where I gained my PPL. But the outfit near Kavala in northern Greece where the original poster intends to train should be just as good. I met one guy who did his PPL there and he was very pleased. Exams are in English (Greece being one of the few countries where you can do your PPL in English) and of course so are communications with tower.
Exchanging your Greek PPL for a UK one should be just a simple administrative procedure (with some money changing hands between you and the CAA for a change). In any event, from Beagle's LASORS' quote, it follows that you can have an IMC issued, it will just go in a separate bit of paper and not on your licence, just like a French national qualification does not go on your British PPL.