If this happens, you will move on to a deemed rate tariff or rolling contract, depending on the agreement you have in place with your current supplier.
If you want to switch and your supplier says you cannot, it is their responsibility to explain why this is as quickly as possible. They will also have to go through what options are available if you believe that you should have the option to switch.
According to Ofgem, the independent energy regulator for Great Britain, if you have any issues with your supplier while switching, you can contact Citizens Advice for further guidance.