Take my experiences recently - I subscribed to Sky for their "See- Speak- Surf" Deal.
I was, and always had been, a BT subscriber.
In order to extricate myself from the Broadband provision it was necessary to obtain the MAC Code from BT {something to do with Migration Access}.
This was required by Sky - in order for them to activate my application with them!!
So within a few working days I was notified, by Sky, of my 'activation date'.
However:-
There is still the need to remain as a BT subscriber in order to avail myself of the 'Sky' Deals.
[The copper wire link from my home to the outside world remains BT's concern, after all is said and done]
Hope this helps.. :-)