Perfectly understandable. I would do the same however, it it dont to whay it says on the tin, then should you or for that matter pay hard earned £££ for inferior quality and connectivity?? Yours to decide.
Virgin Media operates its own speedtest sites however, its not open to public access...surprisingly!!
I would suggest this one, I used it to dispel some of the puffery from VM customer retentions/technical support bods. http://www.speedtest.bbmax.co.uk/
Pity they haven't updated the web-page so that their speedometer reads above 20 Mbps for downloads!