When choosing a UK casino with many high RTP slots, look at the game library...

https://wiki-global.win/index.php/What_Should_a_%22Secure,_Regulated_Platform%22_Feel_Like_as_a_Player%3F

When choosing a UK casino with many high RTP slots, look at the game library size and the software providers offered. RTP shows the theoretical return over time but doesn’t guarantee wins