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

https://alpha-wiki.win/index.php/PlayOJO_Casino_%E2%80%93_What_Does_%22Transparency_and_Simplicity%22_Mean_in_Practice%3F

When choosing a UK casino with many high RTP slots, start by checking the game library size and software providers. Look for filters that let you sort slots by provider or category