I've found it really varies, depending on broadcast.
What's more gobsmacking is that so much BBC output still isn't HD (Doctor Who springs immediately to mind), yet Last of the Summer Wine is. Yes, really. Guess it's got an export market to the US?
Another HD sound issue to listen out for is that on Sky Anytime - sometimes the programmes listed as from Sky One HD etc have inferior sound to the actual shows when broadcast/recorded. Definitely the case on 24....
Clare Newsome is Editor-in-Chief of What Hi-fi? Sound and Vision