Greece
This is exactly the sort of thing I like bringing back from Greece: small enough to fit in a bag, but specific enough to remind you where you bought it. Oils and vinegars work well when the bottle is small, sealed and wrapped in clothing inside checked luggage. It is also the kind of thing you can talk about when someone asks what you brought back, because there is usually a little story attached to where you found it.
Greece
I would put Greek mountain tea on the list for Greece because it is the kind of thing that feels useful once you are home, not just another airport purchase. It is a good one because people can try it slowly at home, and it makes a nicer gift than a mug with a place name printed on it. The reason it works is that it gives you a quick reminder of the trip without needing to keep a fragile ornament safe forever.
Greece
Purple-brown Greek olives with a rich, fruity flavour.
When I think of an easy, crowd-pleasing thing to bring back from Greece, mastiha liqueur is a good pick because it does not need much explanation when you hand it over. For alcohol, I would only bring this back if it is sealed properly and comfortably within the UK allowance; otherwise it is better as a UK-buy link. I would rate it higher when it is locally made, clearly labelled and easy to use at home, and lower when it is just tourist packaging.
Greece
Spoon sweets is worth adding because it feels like something you actually noticed while travelling through Greece, rather than a generic souvenir. Look for a boxed version with a decent date on it, especially if you are buying it near the start of the trip rather than at the airport. The ideal version is modestly priced, not too bulky, and good enough that you would buy it again even without the holiday memory.
Greece
Blue glass charm used across Greece as a symbolic protection gift.