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

https://super-wiki.win/index.php/If_I_Want_High_RTP_Options,_Should_I_Pick_a_Casino_with_Hundreds_or_Thousands_of_Games%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 number, so look for casinos that offer filters by provider and category to easily find slots that suit you