I was thinking of taking Sky line rental a few months ago (I didn't in the end). As I wondered about moving back to BT when on Sky line rental, I sent BT an email.
Me: I am currently paying Sky for my phone line rental. If I move my line rental to BT will there be a connection charge, and if so how much is it? Thanks in advance.
BT: I confirm that if you will transfer your services from Sky to BT then there will be no connection charge and the services will not be interrupted as well because it will be a take over of the services, If you wish to make the arrangement then you can directly contact our Customer option team on 0800800030 they will place the order for you.
OK you can believe BT customer serivces if you like.

You will find that if you call that number they will change their tune to "Oh we thought you mean Sky connect (in which case they would be right in what they said)
I can assure you if you leave Sky LLU you will be charged and lose service (I say this as I sit here using my tethered mobile phone for internet access - just like I have for the last 2 weeks). If you like chatting to customer service, maybe ask Sky themselves. To save you time, I did so myself on their live chat service:
You: If I join the Sky Broadband and Sky Talk phone service, and then in the future want to move back to a BT service, will there be a connection charge, and there be any interruption of service?
Jordan: there can be interruption of 2 weeks in you are in Sky broadband Area and BT may charge you for that
You: And will both the phone and internet be interrupted?
Jordan: yes both phone and Broadband will be affected
You: Will i lose my telephone numberif I go back to BT?
Jordan: no, you can still keep your number
You: Ok thanks for help
So I repeat...if you leave Sky LLU for BT or another BT compatible service, you will almost certainly be charged anything upto £130, and you will lose service for a minimum of 2 weeks.