I hate to bump, but I found a solution that at least works for me. Instead of going to www.virginmobile.co.uk/getmessage (which, incidentally, still doesn't work), try going to https://www.mms.t-mobile.co.uk/mmcp/Render/index.jsp instead. Yes, I'm with Virgin, but bizarrely enough, I can retrieve my picture messages there fine. They don't display in the browser correctly, but if you use the forward to email function, it should work.
Of course, it COULD be that since my number was transferred from T-Mobile (I believe, not 100% sure who I was with previously), that somehow their portal is still accepting my picture messages. Either way, using the T-Mobile MMS retrieval thing works. I'm going to call Virgin tomorrow and point this out to them, because it being down for what must be at least a week by this point is just unacceptable.
I don't work for Virgin, and I may be slightly mad. Take my advice at your own risk.