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

https://trevorssuperbnews.hexaforgey.com/posts/how-do-i-compare-sports-coverage-across-bookmakers-quickly

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.