Most suppliers charge an annual fixed fee for the administrative costs relating to an energy contract. Some suppliers charge this fixed fee for each year that you begin as a customer. Others calculate the fixed fee on the number of months you were a customer (‘pro rata’). This is completely independent of the duration of your contract (indefinite or fixed term).
You should certainly take this into account if you want to switch supplier. If you currently have a contract under which you must pay the complete annual fixed fee when you switch to a different supplier, have the date of your new contract coincide with the end date of your old contract. For example: if you have a contract that runs until 30 April, you should have your new contract start on 1 May.