Sky is LLU, however they do accept MAC codes.
Who are you with now? If it is someone with BTWholesale then there will be a cease placed on the BTW service before Sky can take it over. However as Sky accept MACs your downtime could be as little as a couple of hours. If you are with another LLU provider then you will go through the same process (unless it's easynews) and in theory the downtime shouldn't be any longer, not sure if it actually works out like that.
As for service, I have read some very mixed reviews. The consensus seems to be that you simply cannot get an unlimited 8meg connection for under £25/month, everything else has quite severe traffic shaping of some sort, so if you are a heavy user then maybe do some more research.
