When choosing a UK casino with many high RTP slots, look at the game library...

https://iris-wiki.win/index.php/Why_Do_Bigger_Game_Libraries_Matter_for_RTP_Hunters%3F

When choosing a UK casino with many high RTP slots, look at the game library size and which software providers are featured. RTP is theoretical, so use filters like provider and category to find slots that fit your play style