Originally Posted by AJA_

Car hire has to be booked at the same time as the flights via the BA Holidays part of the website. If youve already booked the flights adding a car subsequently will not work.

Also the T&Cs were very recently changed such that the car hire or hotel must be for at least 5 nights with the flights at either end. You also are not supposed to book flights to one destination and car hire elsewhere.

Edit to add: November 2023 is still fine though. Offer currently runs to 31 Dec 2023 though its not clear if return travel has to be done by that date but I suspect it is travel completed by date