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

https://wiki-stock.win/index.php/How_Do_I_Check_If_a_Slot_Site_Is_Really_UKGC_Licensed%3F

A UKGC licence is the baseline, not a guarantee of a smooth experience. Check for clear, game-level RTP information, realistic withdrawal times and useful responsible gambling tools, including deposit limits and time-outs