Check each site's licence on the UK Gambling Commission register, then read...

https://ricardosinsightfuldigests.huicopper.com/what-should-a-reliable-uk-betting-site-ranking-include-besides-bonuses

Check each site's licence on the UK Gambling Commission register, then read bonus terms: licensed operators face a 10x wagering cap. Treat headline offers sceptically, and test mobile usability and withdrawal options, fees and times before signing up.