I wonder how many aviation people are as surprised as I am to learn that a B747-400 still uses an oxygen ring-main for the whole cabin
I think the reason is extended operations over the Himalayas. A lot of B777 have oxygen generators, but the BA aircraft have oxygen bottles for the same reason. Oxygen generators have a limited life. I believe the best is about 30mins. This means you have 30mins to get down to 10000ft, and can impose flight planning restrictions over a large mass of high ground.
I know that when our B767 (with generators) fly certain routes they have to carry loads of portable bottles for this reason.