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

https://andressinspiringthoughtss.theglensecret.com/what-does-gambleaware-say-about-using-rtp-to-pick-games

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