UK online casinos operate under strict rules from the UK Gambling Commission...

https://businessmodelanalyst.com/the-uk-online-casino-business-model-revenue-streams-regulation-and-competitive-dynamics/

UK online casinos operate under strict rules from the UK Gambling Commission that aim to keep play fair and safe. For example, many slots offer an average return to player (RTP) of around 96 percent, meaning the house margin is built into the games