Can you provide a link to this Sky+ HD offer, because you may find it works out a bit more costly after the initial contract, unless it 'rolls' on (as HD normally has a 10 pounds per month surcharge over whatever package deal you have).
It does sound good (hence my interest in the deal) but I would tend not to go for them having the line rental, as telecomms and broadband suppliers face fees going to/from BT line rental, and usually pass those fees on. So even if they absorb the fee for switching you to Sky, you may at some future time have a fee to pay to switch the line back to BT, and a possible interruption to broadband and phone service.
In terms of faults on the line, knowing I'd talk directly to BT would give me a bit more confidence than someone from another firm such as Sky, who then passes the details on - you have lost the timestamp of when Openreach are told about the fault. (Also, even though Openreach is meant to treat them all equally, I suspect BT gets a tiny amount of priority from Openreach).
Also, keeping with BT means one could get the 10.00/month line rental (by paying a year up front), and divert calls through cheaper services such as 1899.com
However I'd consider the Sky option as (a) they'd add a dish and do the installation, and (b) after the first year, I'd cancel the service, and use a FreeSat HD box. Sky would probably offer me some deals to keep me, eg "FreeSat from Sky" - so I could switch back to Sky later, or if I cancelled and said I was getting FreeSat boxes anyway, they may later offer inducements to return.
I was with Sky in the long-distan3t past (10+ years ago) first before the DigiBox came on the scene, when they had 60cm and 80cm dishes. Anyway, I was given 3 months of movie channels free on Christmas (when I was unemployed, so used it to the full and was a bit of a hermit while everything froze outside!) and later, offered all channels for a year but with me paying only half the rental (17.50 a month, yes it was a long time ago!)
With a bit of extra kit and 2 more cables from the dish, it would be possible to add boxes to other rooms (3x bedrooms) at limited cost (one off for parts and cable), where Sky would charge 10 quid per month per room.
Does seem a generous deal - are you in an area which is being switched to digital ? If so, unfortunately, I'd probably be ineligible (I missed out on the offers at the time this region was being switched).