The requirement for upright seating is simply to ensure that in the event of a crash or sudden stop, that the seat belt is effective. If you are reclined, you can 'submarine' under it and it can slip to a point where it can cause you serious injury.
Virgin UC pax are allowed to have their seat in any position because the seat belt also contains an airbag. BACW have the seat sslightly reclined as this is the most effective angle for the seatbelt.