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

https://reliabless.com/rtp-vs-volatility-which-matters-more-for-a-short-slot-session/

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