Some suppliers will charge you an exit fee for leaving your contract early but an important note on this — if you apply to leave your old supplier within 49 days of the end of your contract, they are not allowed to charge you any exit fees. You can see a more detailed explanation on the energy regulator OFGEM's website. The bit you need to know is highlighted below:
"Suppliers will be required to notify customers that their current fixed-term is coming to an end between 42 and 49 days before the contract ends.
Between this notification period and the end of the fixed term contract, suppliers will be banned from charging a termination fee should the customer decide to switch."