I only have knowledge from my experiences and I have always used Mac codes when switching LLU provider, never had any problems getting them and have always been asked for by the new providers
the broadband service is supplied through a full Local Loop Unbundling "package"
Perhaps by package they mean line rental and broadband in which case that is true (as in my previous post), I've always had BT for line rental and LLU broadband providers since they first came available, until switching to infinity.
I moved from Sky to Infinity a few months back - I had an LLU connection (not Connect) but the phone line was still BT's (if that makes sense). I requested the MAC from Sky and was given it on the phone during the calll (they asked why I was changing and offered no alternative) - I seem to remember you have 28 days to use the code, if it is not used, everyting remains unchanged.
I ordered Infinity with the code, and the order went through as expected, I was 'offline' only for the amount of time the engineer was here installing the modem - it was very smooth in my case.
Not many people come on here to post poitive things about Infinity, but in my experience the transfer was easy, the connection has been very good and consistently fast, and the HH3 is 'miles better' than the old white Netgear router that Sky gave me. I'm sure that there are better ones, but for my use it has been OK other than an irritation when we have a large number of laptops in the house (>10), but the mods on here are helping me out with that.
Hope that helps and that things go well.