Assuming you've rebooted your box once, pixellation is nearly always the symptom of a problem with your digital signal. It can be easily rectified by reporting the problem to Virgin and getting whatever is causing that problem, fixed.
As a further diagnostic, BBC1 HD shares a broadcast frequency with the following channels. Depending on your package, can you check these channels:
BBC2-HD, ITV SD, BBC2-Wales-HD, BBC-1-Wales HD, Liverpool TV.
If any of those channels are also affected, that is very strong evidence of an issue with your incoming digital signal.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks