UK online casinos operate under strict rules set by the UK Gambling Commission,...

https://wiki-view.win/index.php/How_Do_Casinos_Measure_Whether_Withdrawals_Are_Hurting_Retention%3F

UK online casinos operate under strict rules set by the UK Gambling Commission, which means they must be transparent about game odds and payouts. For example, many games offer around a 96% return to player (RTP)