The Astra 2 satellite cluster offers programming from many sources. The 20 or so HD channels on Astra 2 are not all Sky branded, some are pay channels via a Sky subscription and some are FTA. However I believe they all offer 1080i as their broadcasting standard and produce good results on HDTV sets. I can't comment on what standards Virgin offer but it sounds as if they aren't as good as satellite origin material.
Take a look at C4 HD as they broadcast all C4 output in HD even though some of the programmes are SD origin. C4 HD evening news is a good comparison as the studio is HD but often inserts are in SD and you can see the difference.
Also have a look at the early evening FIVE aussie soap which is originally in HD but broadcast by FIVE in SD but at a high bit rate and looks much better than the usual SD output from the BBC.
ITV often transmits its programmes in SD at lower bit rates and you can also see the difference with poor picture quality.
If you want to watch ITV's few and far between HD programmes you will need a 'freesat' badged box as ITV HD can't be viewed by the 0.5 million Sky HD boxes (so they are only seen by a tiny few thousand who own 'freesat' HD boxes).