There are two problems causing the picture quality issues.
First, channels like BBC News 24 are compressed too much, and look crap. Posher channels like BBC1 and C4 don't compress so much, and the picture quality is better. Incidentally, the same compressed picture is used on all digital platforms: Freeview, cable and Sky, so no platform is better than any other in that respect.
The other issue is the STB. Most Virgin ones, especially the old Pace ones, are crap with very poor picture quality. If you have a Pace box, complain and ask for a newer Samsung one.