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

https://wiki-club.win/index.php/Does_a_Higher_RTP_Actually_Mean_I_Will_Win_More_Money%3F

When choosing a UK casino with many high RTP slots, start by browsing the game library size and checking which software providers they offer. Remember that RTP is a theoretical payout rate and not a guarantee