Shot one:-
PS, chugalug, I don't hold up the 50's/60's UK airlines as a model for optimal procurement.
Well neither do I, it was merely a personal anecdote to confirm riff raff's point that commercial aircraft were bespoke then and that they are bespoke now.
As I've said above, COTS has its place just as bespoke design military equipment has. If both are done well then our Armed Forces can be as well equipped as they can reasonably expect within the financial constraints that the country is faced with. The operable word of course is "If". Such a small word, such a big ask...