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

https://extra-wiki.win/index.php/RTP_vs_Volatility_-_Which_Matters_More_for_a_Short_Slot_Session%3F

When choosing a UK casino with many high RTP slots, start by browsing the game library size and software providers offered. RTP is theoretical and doesn't guarantee wins, but larger libraries usually have more variety