The Trident vs Boeing 727 and RR Medway vs RR Spey vs P&W JT-8D stories are quite complex and touched on in the current Aeroplane Monthly and the Key Trident Special...at one point De Havilland had hopes of a collaborative tri-jet program with Boeing...... a 12,500 lb thrust Spey built by Allison was a Boeing early alternative to the P&W JT-8D on the 727.... the RB141 ,which was running late 1959 was proposed for a Caravelle 8 and for later Qantas short-body 707-138s at 15,000 lb thrust (Google books)....The BAC 111 did brilliantly selling to American and Braniff but the program seemed to stall....maybe George Edwards got disheartened after TSR2 was cancelled, maybe they should have put the JT-8D on the BAC111 like Sud did on late Caravelles.