Thessaloniki is generally considered to be the cultural capital of Greece. It's served by direct flights from Gatwick (BA, TUI and easyJet), Luton (easyJet and Wizz), Stansted (Jet2 and Ryanair), Manchester (easyJet, Ryanair, TUI and Jet2), Birmingham (Jet2), East Midlands (Jet2), Edinburgh (Jet2), Leeds/Bradord (Jet2), Newcastle (Jet2), Birmingham (TUI) and Bristol (TUI), so it's easily accessible from most parts of the UK.
See here:
https://theculturetrip.com/europe/greece/articles/10-awesome-reasons-thessaloniki-should-be-on-your-bucket-list/
Official website here:
https://www.thessaloniki.travel/en/