Originally Posted by Gardyloo

It's probably not taxes, it's fuel surcharges being added by some carriers, most notably British Airways. Reducing the number of BA segments is one way to lower the "tax" cost. This makes South Africa hard as an origin/destination point, since for the moment BA is the only game in town for European arrivals/departures from JNB/CPT. Cathay may also be adding fuel surcharges (look for "YQ" in the tax/fee breakouts) so you might substitute AA for the transpacific segments, e.g. HKG-xNRT-DFW.

In addition, stopovers in the UK will result in very big UK Air Passenger Duty charges being added. These can be avoided by only transiting through the UK, or the amounts can be reduced by limiting UK departures to short-haul destinations, as the APD is distance-based.