Well I am afraid the nature of capitalism is that BA would like to make money. Doesn't always work to one's own advantage unfortunately. However, why did the agent not split your booking out, thereby saving at least the service for the non pregnant passenger? Or was this never going to be a one person trip?

I had a case recently where I booked a reservation for me and a friend on a very hard to get route. I made the booking unsure whether my friend was available, and he's notoriously tricky to get hold of. So I booked it anyway and thought I would cancel if needed, which I can do for free. As it happened he did respond to my text almost immediately, with a negative. No problem, I cancelled his ticket within 5 minutes of the booking. That seat did not go back into Avios inventory that day - but availability did open up the next moring. So keep checking.