Asking for a friend :
He has booked an economy ticket BHD-LHR-Doha-Entebbe on ba.com. The Doha to Entebbe flight is Qatar. The economy ticket only has 1 bag. He would like to add another bag, and MMB says “not available” under extra baggage. He has tried calling both BA and Qatar and both have said it can only be done at the airport.
The BA website baggage cost checker quotes £80 “online” and £95 “at airport” for a LHR-DOH extra bag, so it appears it should be able to be booked online in advance and should be cheaper. I was not sure of this price, as it is a 3 flight ticket, but thought that the longest leg might be what defined the price?

Are the phone agents correct? Is there a way to add another bag in advance for lower cost? (Also he is concerned the check in agents at Belfast might also not be familiar with what to do and could refuse). He doesn't want to turn up with three bags and be told he can only take 1.

Also for hand luggage – I presume the LHR – DOH leg is the longest so BA hand luggage rules would apple for the whole flight? He could then take 2 pieces of hand luggage rather than the 1 piece Qatar allow in economy?

Many thanks,
Chris