The answer is completely available by using the on-line booking tool, and going all the way to the pricing/confirmation screen. Once you see the fare, there's a clickable link to a screen itemizing the taxes, fuel surcharges, etc. - specifically for your itinerary. Originating in Uganda means BA as the issuing carrier UNLESS you are flying directly to N. America and can use the AA codeshare on the BA flight to LHR (in which case having the ticket issued by AA will save considerably on fuel surcharges, but will require you to communicate with AA in order to get them as the issuing airline.)

Without knowing precise itinerary, including which carriers on which segments, which points are stopovers v. connections, dates, a guess is all you're going to get here. My guesses - with a lot of BA or QF segments, London stopovers, etc. add 15% - 20% to the ticket cost. With more AA segments, less, maybe 10-12%.

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