I remember VA offering some decent offers on miles boosts in the past with extra miles on top of the purchased ones. Have these completely dried up now? I only see bonuses on miles purchases these days.
All bookings will have access to a x1 or x2 Points Booster option (even reward flights). Occasionally there are bigger booster offers on top of those though.
You need to view your booking using the website then just click the 'Boost your Virgin Points' link. All 3 of my current bookings show this link.