[chickendippers]

The problem with adding your own cable is that it will affect the signal level; you may find that your V+ box won't work in the new location and secondly it may introduce noise into the network thus affecting your neighbour's service.

This is why the recommendation is to have Virgin Media come and do it as the engineer will check the signal level and make the necessary adjustment (in the street cabinet if necessary) for you. However IIRC they're not permitted to go into loft space.