Originally Posted by
Steve M
...If you find yourself in such a situation (well, really anytime you get downgraded, regardless of whether there was an upgrade opportunity), and you have the time flexibility and there's another ow carrier that can get you to your destination....
Only a very few routes have more than 1 Oneworld carrier, but you often can change the date. The majority of city pairs served by Oneworld have precisely 1 carrier that can be used on OWE ticket. JNB-LHR certainly falls into this category. Also OP was probably flying the first sector, which has special rules (any changes require refund of the ticket and buying a new one, so not really an option in this case, and BA knows it).
My own guess is that the maximum compensation you can get these days is 75% of the sector purchase price (the OWE addon sector cost); and even that will rarely be offered. The airlines could claim that this represents the cost of the single sector (not unreasonable, as it is close to the 1/16th price of the ticket) and courts would accept that. Though discovery to determine how the OWE fare is allocated would be fun.