Exactly right .
If you call them they can talk turkey. They cannot initiate anything except they have to let you know what charges (if any) you will incur from them on leaving.
You have 10 working days for you to contact your new ISP and cancel. In practice that is 10 calendar days as two or three days before the move date it gets too late to stop the migration at the wholesale/Openreach levels.
The only two (IIRC) times your current ISP can cancel/stop the migration is if you are being slammed by the new one, or when you tell the new one you wish to cancel and they refuse to accept that.