There is no 'jumpseating ticket' for FOs. You buy an ID90 or you don't get on the aircraft. Then you might have to sit on the jumpseat anyway if the aircraft is full of commercials.
If you want to get on the aircraft you need to have bought your ID90 and listed for the flight an hour before. A new system for allocating jumpseats has been introduced and you will now have to ask to be considered for a jumpseat about 65 mins before departure.
There is no such thing as a sure deal on BA staff travel.