Many punters in the UK have accounts with multiple online bookmakers to compare odds, markets and claim several new customer offers. Besides this, being signed up to more than one betting site gives you peace of mind. For example, if your sportsbook experiences technical issues, you have another option to use straight away.
Joining more than one - or even a handful of betting sites - is recommended. Bear in mind, though, that certain betting companies operate several betting sites and sometimes limit new customers to one welcome bonus for their network of sites (not one per site).