Best value for money - Hmmm. Depends on how much you want to spend I suppose.
Cheapest is freesat as it doesn't cost anything after you've bought the kit. As for a fair amount of HD, that's pushing it a tad. I get BBC HD and the occasional footy match on ITV HD. I think there are a couple of extra channels you can get but not too sure how to access them.
I also have Virgin TV (XL) and bundle it with my broadband (10mb package). My bill tells me this saves me 9 quid a month. In addition, Virgin ring me regularly to inform me that as a valued customer, I qualify to have free installation of their telephone lines, to which I always reply "thats jolly decent of you but no thanks"
The on demand TV services from Virgin are very good with XL. You get lots of general TV, series catch up, music videos and on demand films which, dependng on the film, can be between free and £3.50 a pop. I don't have a V+HD box because I'm too tight to shell out the £75 they want to install it - strangely enough though, as a valued customer, I don't qualify for a free install!
I used to have sky many moons ago and still would have if my phone line were better and I could get a decent Broadband service. I'm not really a sky fan to be honest but I just think they do TV better than anyone else. Their HD content is second to none, the Sky+ system is very good I'm told and I always found them pretty good to deal with.