Should be outlined in the existing providers terms and conditions.
There is no 'cease' charge levied by the big providers where people are migrating.
Unfortunately, the terms and conditions can be sufficiently vague to give little warning of whether a cease charge will be be imposed.
In my case my FTTP service was out of contract, and I took out a new contract with a different provider for a rather better price. Usual practice is for this to be treated as a migration, but my soon to be former provider insisted on treating it as a cease, and charging for the privilege ... not happy, but I couldn't budge them.
Fortunately, the new provider's price still offered a cheaper annual cost, so I came out ahead. The catch is that with a cease, it can take several days for Openreach to make the switch to the new provider, leaving you internetless in the meantime. Fortunately again, my new provider (Aquiss) was very hot off the mark and got me up and running within 15 hrs ... happy!