I'm having the same issue. Noticed a lot of other posts seem to be going up as well about this problem.
Calling Virgin they advise it's not an issue their end. I began having problems about a week ago. I visited my mum at the weekend and her BT connection streamed Youtube videos without any problems. I return home on Monday and I am still having the same problems.
Tried various devices, re-booting the router etc. The videos stream fine in HD over 3G on my mobile network. It's defnintely something to do with Virgin.