You might want to look at the terms and conditions for the promotion and the postings in the thread devoted to the promotion here:
https://www.flyertalk.com/forum/brit...kings-143.html
In my opinion, it would probably run afoul of this provision:
2. For the avoidance of doubt this offer does not apply to any British Airways flight only bookings made without a hotel or car or where the hotel or car is added to a booking in a separate transaction.