Took a little hunting but I found this picture.
From which it seems you have 5 ways to connect.
1) With RF cable of course that will not be HD.
2) Component Video again not HD
3) S-Video not sure on that but don't think it's HD
4) SCART again don't think that's HD
5) HDMI now that is HD
So to answer you don't NEED a SCART lead there are 4 other methods and I would expect local TV would use HDMI and remote TV's the RF output with DVD recorder using SCART that's how I would set it up.
I see it has 5.1 surround sound not sure how you would connect to that? Likely with the Optical Audio out!
There are three reasons to get HD box and main must be to watch programs in HD and as far as I am aware to do that you have to use HDMI lead.
Second is to get a larger capacity hard drive although since you have to pay Sky to use drive I would prefer to transfer to a medium which will work without subscription.
Third is to watch some of the programs which are either shown at different times on HD or not shown on standard system.
The first surely must be main reason? So why question about SCART lead?