BA are quite clear in their terms. If you book less than 72 hours before departure there is no exemption from APD for minors:

Air Passenger Duty (APD) charges for 12-15 year olds

Air Passenger Duty (APD), a UK government tax on flights departing the UK, changed recently for 12-15 year olds travelling in economy. Depending on your type of flights and when you purchased them, you may be eligible for a refund. If you are eligible, you must take action at least 72 hours before you travel to qualify.

I don't have a breakdown. I only have the e-ticket receipt which shows a fare (for 2 pax) of £626 and a total charge of £777.06