I was pretty confused when I first looked into it. LLU seems to come in a number of flavours, including "full unbundling" and "shared access". I'm with TT, who take over the responsibility for operating your line, your voice service and your broadband service.
Sky do the shared access thing, where BT still operate/maintain your line, but Sky provide the services such as voice and bb. You can pay Sky your line rental, but I think it goes straight to BT anyhow?
With full unbundling, if you have a fault on your line and you phone BT, they'll tell you to naff off
Well that's the understanding I came to, it's probably wrong
I found a diagram to help confuse you more
http://www.ofcom.org.uk/static/archive/oftel/publications/broadband/dsl_facts/LLUbackground.htm