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

https://shed-wiki.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 checking the size of the game library and the software providers available. Look for filters that let you sort by provider and game category to find slots with higher RTP percentages