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

https://wiki-wire.win/index.php/What_Are_the_Most_Common_Complaints_About_Online_Slot_Sites%3F

A UKGC licence is the baseline, not a guarantee of a smooth experience. Check that RTP information is easy to find, withdrawals are processed promptly, and responsible gambling tools are simple to use