This is during the period when it is commonly stated BR were "starved of investment". In this time, starting in say 1967, they introduced air braked Mk2 stock, air con from the 1971 build, which within 10 years had completely taken over Inter City services. Carried on for the next 6 years or so and cascaded it all further with HSTs. Derby works were just churning them all out, at considerable cost.
I believe BR were the first European railway to change their volume vehicle production over to air con for all classes.