Cares harness is minimum 22lbs which is about 10.5kg and I'm not sure it would work well for a 9mo. In fact I don't know if you're even allowed to use them for children under 2. (I just checked Virgin's site and they say it's for children over 2 only).
I'm pretty sure that for regulations (governing all airlines):
Infants under 2 must be on a lap with the supplementary belt OR in a car seat whenever the seatbelt sign is on.
They can only use a car seat if you've bought them their own seat to secure it in, and brought an appropriate seat.
If you turn up and they deem the seat inappropriate, they may let you use it on the airline seat but they won't let you use it while the seatbelt light is on - baby will need to be on your lap. They may insist the seat goes in the hold so it's not a projectile causing danger for other passengers.
If you turn up and can't get the seat secured or it doesn't fit within the airline seat, they will put it in the hold and you will have the baby on your lap when the seatbelt sign is on.
In terms of Virgin specific policy: I've checked their site and there is on;y reference to "can" use a car seat rather than being "must/have to". Since they don't allow rear facing seats, it seems logical this must be the case, as you can't even buy car seats that forward face for very young babies.