According to Ofcom, if you are still within a minimum term contract and use OTS you still have to pay fees for the remainder of the minimum term.
/ofcom.org.uk/phones-and-broadband/switching-provider/switching-broadband-provider
If you want to change provider before the end of your minimum contract period, you might have to pay early termination charges (unless you're not getting the speeds you were promised). If you use the OTS switching process you will be automatically notified by your current provider about any termination charges.
And remember, you have the right to cancel your order within 14 calendar days (the ‘cooling off’ period). However, if your service has already started then you may be charged for the proportion of the service you have already used when you cancel and/or any installation costs incurred