Switching suppliers is always free of charge. However, you should make sure that you respect the contractual clauses regarding the period of notice, especially if you have a fixed-term contract, in order to avoid possible penalties.
If you are charged a change of supplier fee, you should pay the amount claimed under no circumstances. In such a case, you must dispute the bill, preferably in writing, with the issuer. If, however, your complaint does not result in the bill being cancelled, you have the right to refer the matter to the ILR within the framework of a mediation or complaint procedure; you will find a specific tab on this issue on the ILR website.