Ash,
You have answered your own question. When you buy a business class long haul with KLM (or its partners) or even a european onward connection from AMS in business using a UK KLM feeder ie from NCL you are usually in business on the UK sector also if that makes sense.
The same applies on the LHR domestic flights with BA and its partners.
I seem to remember someone saying that its to give a seamless business experience for the entire journey ! ie lounges, priority boarding, extra bags etc. But obviously not the same seat.
So perhaps the vast majority of those in business on NCL-AMS and NCL-LHR fit into the above category.
Most of those in business on the feeders will therefore not be paying the high prices you quote ( a few will though )