"No it's not fibre optic to the house it's fibre optic in the street and the usual telephone wire to the house."
well that sounds like an unbundled BT Infinity service - except
AFAIK BT haven't let anyone else resell Infinity yet.
All the pointers are that you simply have an unbundled ADSL service through Virgin using BT's local cables. NOT a Virgin cable service
In which case you do have all the contention ratio issues to play with.
And one final point - you're trying to download Sky Go through a Virgin network??? One suspects you may be falling victim to some kind of traffic profile shaping or prioritisation. Internet companies hate streaming services, and especially those of their competitors. It would not surprise me if streaming Sky video had a low bandwidth priority for Virgin