Just to update this thread.

I phoned and initially it was as I expected and the agent was able to sort out the booking as I wanted for a total cost of 20000 Avios and £244 taxes + £15 booking fee per person.

However when it came to payment they were not allowed to take payment over the phone because Sierra Leone is flagged as a fraud risk. So the options were to pay online (which wasn't an option as per previous problem) or to pay at a BA travel shop in person. So it was a trip to Glasgow Airport to pay with the booking fee being waived. Luckily I was passing the airport so it wasn't as much hassle as it normally could be.