Yes. For energy and broadband you have 14 days to change your mind. They start the day after your contract with your new provider is made, so if it's made on 1 March, you have until the end of 15 March. Your contract can be made before their welcome or confirmation email arrives, and the 14 days don't wait until the switch happens. If your provider gives you longer, you can use the extra time.
Energy: cancel within the 14 days and your new supplier will stop the switch, so you stay with your current supplier. If the switch has already gone through, ask your new supplier what happens next: you may need to move back to your old supplier or choose another one. You'll usually pay for the energy you used before you cancelled, but only if you asked for your supply to start within the 14 days and your supplier told you about your right to cancel and that you'd have to pay.
Broadband: your new provider stops the switch. If it hasn't happened yet, your old service carries on. If your contract includes equipment that becomes yours, such as a router you keep, the 14 days can start the day after it arrives instead.
To cancel, contact your new provider, or email [email protected] and we'll ask them to stop the switch and let you know what they say.