Thinking as a novice.
Would I use it if it was free. Yes.
Would I use it if it was a subscription, possibly if it was sub £3 and included in the BB package.
" Is there one overriding thing apart from the cost that would prevent you from subscribing? "
I am surprised the package does not come with a pro version of a reasonable AV. If it was bundled with say Norton, Kerpersky or similar. They could offer a package that gives you peace of mind that your computer is perfectly safe and if you encounter hardware or software problems you have phone or email backup from advisor's. Virgin could make a excellent deal with an AV company and then push up the monthly subscription to £6.99 giving them a yearly subscription of £83+. What would be the cost to Virgin? £10 to £15 per unit?
Look what tech friends offer click here Most novices would be tempted with this when they purchase their new computer. They sell peace of mind at the point where most novices as for advice. Virgin will have to break into this market, by aggressive advertising on TV, newspapers and their home page website.