Old provider is shell broadband
There may be an issue there, as FTTP does not use the Openreach copper network, so I am not sure what happens.
It may be best to contact Shell broadband yourself, and tell them which day you want to cease your copper service with them.
Your switch could be a little more involved given that you already had a migration in progress, you were Shell to SSE , if you have cancelled with SSE , then contacted BT , if the SSE order hadn’t been dealt with as a cancelled order by SSE , you would think the order with BT wouldn’t proceed ( there is still time for you to be contacted by BT stating the existing order needs to be cancelled and a new one raising ) , but otherwise it may well proceed.
Another potential problem ( as already stated ) is if the BT order is FTTP , and the Shell to SSE was on copper, unless you asked for the phone number to be ported , then the Shell service ( assuming the SSE order is cancelled ) may continue , although not normally a good idea to contact the losing provider ( Shell ) , I would consider asking if they have been advised you are leaving them
The SSE seems to be cancelled however it’s referred back to shell but they have cancelled so it’s gone back to sse who have literally just sent me an email setting up date 5th jan BT is engineer on 6th jan for setup on 7th. It is all full fibre yes. So I could possibly be without broadband couple of days.