This is a very relevant topic for the BA forum because it was only in the last 20 years of Kai Tak's life that Cathay Pacific rose to become the number 1 operator there, before that, and for the bulk of it's time open, it was BA, and of course their predecessor BOAC. Even into the 1980s the Hong Kong to Johannesburg flight was still on BA, and they had the monopoly on Hong Kong to London, while slightly earlier they had given up the daily transpacific route through Tokyo and Honolulu to San Francisco, and handed over the remaining flights to Sydney (which were theoretically through from London) to Qantas.

As I understand it they would have profitable under normal costs, but BOAC/BA crewed the entire operation from London which cost a fortune in crew accommodation etc.