OAF was a standard BA machine, except that being younger it (like OAG) didn't have the 'crown area' mods done
OAF was indeed a true BA aircraft, it title was bought for £1000 and 10,000FF, but significant sums ( million+) were then spent to deliver it to the BA spec. BA did fund and purchase it as their 6th Concorde, albeit at a greatly reduced price.
OAG sort of fell into BA's use. They had "bought" it for under £100,000k, with a must sell back clause , before OAF was delivered to give their services resilience while OAC whet back to Filton for repair. During its use it suffered contamination of its hyd systems, so was grounded, before this was repaired the Concrde programme all but shut down and BA held onto the aircraft fully registering it as OAG, initially it had been G-BFKW.
OAG flew for a short while but was eventually grounded, as I understand it mainly for spares recovery, but as it was a million miles aware spec with from the other BA ( cabin was even different) it was a easy choice to make.
When BA acquired all the spares and full access to G-BBDG a decision was made to bring OAG as close to BA spec as possible and be the first to have a new interior on the fleet. OAG was then then launch Concorde in the land our livery and a lot OAF was a standard BA machine, except that being younger it (like OAG) didn't have the 'crown area' mods done the air to air shots from this time are of it in 1985.
Crown area mods...were these not mandated and embodied fleet wide at the 12,000 major?