It depends on your contract, and energy and broadband work differently.
Energy.
Fixed tariffs often carry an exit fee, usually per fuel. Standard variable tariffs, and other tariffs with no end date, don't, and there's no fee if you switch in the last 49 days of a fixed deal. You may not be warned about it during the switch, and any fee is added to your final bill, so check your tariff details on your latest bill or in your supplier's app, or ask your supplier, before you go ahead.
Broadband.
Once the switch request is sent, your current provider must send you 'switching information' straight away, setting out any early termination charges and anything else affected, such as a landline or TV bundle. You see this before the switch happens. If it's not what you expected and you want to stop the switch, tell your new provider or email [email protected] and we'll ask them to stop it.