Over-engineered?? Old-fashioned practices?? Not listening to the customer?? All of the above, plus actually listening to a funded but unique customer operating in cartel, pooled revenue, where capacity was driven by a distorted market - not until 1979 were Brits allowed to spend their chosen proportion of net, hard-earned cash in foreign currency. Of Brit jettypes only 1-11 was not launched by monopoly Corporations, but by (Fred Laker as) BUA, ahead of the target of US regionals, who did take it - Mohawk, Braniff, then AA. (to be)BA then launched /500, but by then 737/200 and enhanced DC-9s had left it for dead, because BAC/RR would not punt on upgrades.
There were attempts to do Tay /600, then to retrofit it to /500. Some say that would have cannibalised BAe.146, others that (then-nationalised BAe.) just lost all yen for civil after aborted 2-11/3-11 sorties. Better to make bits for other people. If the purpose of building commercial jetliners is to recover all costs with a surplus, then only recently have some models done this after >1,000 sales. UK thought that pilots/engineers bought airliners. Cost per ASM, maintenance man-hours p flight hour...who he?
Wasn't just Brits. Convair did better building 747 lower 41 and DC-10 fuselage barrels, than on CV880/990. Japan bashes metal profitably.