When choosing a UK casino with many high RTP slots, start by checking the game...

https://shed-wiki.win/index.php/Which_Casino_Is_Best_If_I_Want_Slots,_Blackjack,_and_Live_Dealer_in_One_Place%3F

When choosing a UK casino with many high RTP slots, start by checking the game library size and variety of software providers. RTP is a theoretical return, so use it as a guide, not a guarantee