A UKGC licence is the baseline, not a guarantee of a smooth experience. Check...

https://source-wiki.win/index.php/Which_Slot_Providers_Should_I_Look_for_on_a_UK_Casino%3F

A UKGC licence is the baseline, not a guarantee of a smooth experience. Check that RTPs are easy to find, withdrawal times are clearly explained, and responsible gambling tools make it simple to set limits or take a break.