Indeed the starting point does influence the fare basis (not the class however). However, Q class means something totally different in different airlines, it is the deepest discounted ticket for BA but not SA. In order to make a fair comparison one needs to look at the same fare class for the same airline for the same starting point (i.e. LHR or LGW or JNB - not LON however) in the same season. Including or excluding taxes will depend on your own preferences.
By the way, November is shoulder season hence your Q class fare on SA being much higher than mine. Also your includes all taxes and levies while my data was excluding all!
What you do see in your data is that LAX is a much higher yielding route than HKG for BA since LHR-HKG is much longer than LHR-LAX even though the laters price is the price is only higher by a marginal amount! (For the same fare class).