Ideally, the best time to switch broadband is at the end of your current contract. That way you won’t have to pay any exit fees to end your contract early.
Your provider should notify you when your contract is due to end. You can tell your new provider when you want your new deal to start, and they’ll normally arrange the switch for you.
However, if your broadband is not reaching the minimum speeds you were promised, let your provider know. If their network is at fault and they can’t resolve the issue within 30 days, you can normally leave your contract early without paying a fee.