When choosing a UK casino with plenty of high RTP slots, look at the game...

https://www.random-bookmarks.win/when-choosing-a-uk-casino-with-many-high-rtp-slots-remember-rtp-is-theoretical

When choosing a UK casino with plenty of high RTP slots, look at the game library size and the software providers offered. Remember, RTP is a theoretical return over time, not a guaranteed payout