You are much better to redeem points outright for a flight on Virgin, rather than use to Upgrade a sector from premium to business. But you should also check with an agent what fares they have in premium (that are upgradeable) as a Premium from London to New York can be got for less than €1900 (and upgradeable). However you may have to split ticket which can be problematic should there be any delays.
Either way – save up points and use outright. However if you are a corporate you should also enrol in Flying Co, the corporate scheme. If there are no seats to upgrade instantly on there scheme they will waitlist you and you get it at the airport if seats come free, so much better.