Sky is a great choice. I have just switched to them from Plusnet and not one regret. Support and service both great. Now to answer your questions.
1. No you won't (and btw means BT Wholesale)
2. As you are not switching within the BT Wholesale network, there's no need for a MAC code and you don't even need to tell O2 you're leaving, Sky do that for you
3. Sky does that for you
4. About 2 hours loss of service. They used to switch broadband on around a week after the land line, but it's now a matter of hours
So yes, in your position I'd definitely switch over to Sky.
PS: the above answers are based on the assumption that Sky LLU is available at your exchange
Edited by Xplosion (Sat 19-May-12 21:46:21)