The UK climate makes the olives tough and small. It's the leaves that make the hobby of growing them in the UK worthwhile.
Not having a view on where to get olive oil, I'm happy to add my own recommendation of any oil marked 'Collina di Brindisi' - from the south facing slopes at the south-east corner of Italy. It's a blend. I can only find it in Strasbourg.
Secondarily, I agree with the quality of olives from Cyprus as posted above. Outside of 'thieves pot', there's not much going for Cypriot cuisine, but the olives have a life of their own.
Beyond that, the olives not mentioned in this thread that might seem exotic are those from the Crimea region. But while they might sound exotic, they are actually basic mediocre.