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

https://sticky-wiki.win/index.php/RTP_vs_Volatility_-_Which_Matters_More_for_a_Short_Session%3F

When choosing a UK casino with many high RTP slots, start by checking the game library size and the software providers behind the titles. RTP is a theoretical percentage, not a guarantee of results