I doubt any dealer would take it, registered or not. The demand for right hand drives will be minimal if not zero! Or do we have lefthand drives, oh blimey. No, I am right, I mean correct, lol! I can't even remember what side of the road we drive on, silly isn't it?
Anyway, I doubt they would want one of "our" cars.
Having Greek residency does not mean relinquishing British residency. It just means obtaining a permit which allows you to stay in Greece for longer than 3 months and you need this for various things.