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

https://trentonsbestthoughtss.theburnward.com/what-makes-a-mobile-betting-site-feel-fast-in-real-use

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.