Just seen this post on Sky Chat forum. (Which provider are you attempting to switch to? If you are on Sky LLU, then you will have to go back to BT on a return to donor. It's free, but requring a 12m lne rental contract. You can't switch direct to another LLU service.
You should be able to take your number with you on this basis).
I am confused why you are asking yourself the question of who you are switching to?
In terms of having to go back to BT, that is false, you can switch LLU to LLU, people do it all the time.