This is of a piece with the myth of When Brits Ruled the Air.
The "private" owners of UK aero firms, pre-1977 Nationalisation, were nothing of the sort: this industry did not spend its own money, it spent ours, either (or both) as Treasury Launch Aid (computed as 50% of estimated R&D+Production Investment), and as progress payments on orders placed by BEAC and BOAC, parastatals, whose losses and capital investment fell to us, proud owners. Strong they surely were on techno-innovation; averse they surely were on financial risk. So, to Trident.
The DH Enterprise, and other Britfirms, in 1957 addressed a brewing BEAC Requirement for a prime-pairs jet. DH spread the upfront financial exposure by collaborating with Fairey and Hunting, who would take design and production and cost of defined chunks. BEAC selected their DH.121+RB141 Medway; Airco (the Special Purpose vehicle) and RR applied for Launch Aid, BEAC for fleet capital sanction. During Ministers' review, BEAC revisited forward projections of pax/freight demand: not addressed in any of the we wuz robbed yarns was that happy cartels owned each prime city-pair route: revenue pooling. 9 am departure: BEAC, 10 am departure: Air France; 50% cost, 50% revenue, at fixed, common prices. No competition. Cosy. BEAC scaled down to (what emerged as Trident 1E), aligned to Comet 4B and Caravelle: more capacity would not be admissible in the reciprocal deals.
None of the above applied anywhere else on earth, except Australia. In US, business success of Viscount, FH-27 and (initially) L.188 Electra, led by 1959 to interest in a medium haul jet: the Denver/Rockies Case required in excess of 2 engines; 720 was in hand, but would not offer sensible seat costs on medium haul. So Boeing explored a Trijet, not because DH was already doing it, but because that made sense. It surely made sense to listen to DH's thoughts, but it would not have made sense to workshare, to join Airco, because Boeing already enjoyed all the scale economy they could need, from commonality with cascades of 707/KC-135/B-52. Gorilla, not minnow.
727 had RB141 Medway right up to first sale signature; Pratt bought the job by inventing a fan variant of a military engine and stepping up to its full R&D cost. Something neither RR or DH ever did.
These endless we wuz robbed myths are truly tiresome.