Thanks for all the feedback, much appreciated.

As a matter of fact, I have already spoken to BA but they insist they cannot assign Seats 16 D/E/F because there is only 1 infant in the booking (and they claim 16F is a bassinet seat which can only assigned to a traveller who has an infant in the booking).

I like the idea of reserving D/E/G and then switching with the person in F (if that's a solo traveller without infant - if there is a second infant on the flight, that would be bad).