Mr Mac. Yes, BA have had a near monopoly of this route for decades. They have stayed with it and benefitted from that. Of the a/c that you saw, the 747s are going to LHR and the 777 to LGW. This was a secondary route that they started when someone else tried and then stopped. CPT is mainly holiday traffic and is one of the most sought after destinations for Avios reward seats. Mostly be retired people who can plan where they are going tobe in a years time - going to see the grandchildren!
Incidentally, JNB is the same, you will often see a 747 and a 380 going to LHR within two hours of each other. The 'daylight return' where the a/c turns and goes back in the usual timeframe have proved unpopular as the biz folks want the day for work and the leisure folks want the day for leisure! So the a/c sit for 12 hours and this helps to keep the cost up. Most other European carriers do the same, although some have a daylight return and, if you get the chance to see the Sahara from the air - it is well worth it.