As Lesley said earlier, unless the flight is at 75% capacity they are losing money. So it makes sense to aggregate two flights into one, if feasible. For example, TUI usually have two flights a week from Donny - Monday and Friday. The Friday flight leaves at 7am and the Monday at 2:40pm. Landing fees are probably lower for the Monday flight, so they may decide, if bookings are low, to cancel Friday and offer Monday. Monday is less pleasant for us - so if they cancel our Friday flight we'll have to choose between Monday and postponement.
They have been put in a very difficult position by the UK government's late decisions. People will make last-minute decisions based on the latest travel advice, and they still have to crew a plane fully staffed even if it has only a handful of passengers, because the number of cabin crew depends upon the number of seats, not the number of passengers. The minimum for a 737 is three, but a normal TUI crew appears to be four FAs.
As fas as Spain v Greece goes - Spain has very high infection levels, though not as high as the UK, and substantially greater than Greece. So I think that's a logistics decision, not a pandemic decision. Average flight time to Spain is about 2.5 hours - to Greece is at least three and probably four hours. That's a lot more fuel.