In 1958 UK paid lip service to inter-operation/collaboration, but neither Ministers nor Directors nor Marshals wanted anything to do with Franco/German bids to NBMR.2 (to be Atlantique). Coastal had enjoyed its Neptunes, but "free" (to be)P-3A was not on offer, so a Requirement was invented for 4 engines for UK's blue water role, distinct long range, unlike France's Mediterranean backyard {?Noumea? La Reunion?}. Low-risk derivative was also specified, to further justify inability to "wait" for an all-new NATO camel. So, prop (obvious solution: a Britannia/Argus variant) or turbofan? Spey offered loiter economy with rapid transit, base-patrol, which was presented as fewer airframes to yield 24/7 cover of the 2 lines blocking Sov egress from Murmansk. RR and HSAL then had the sense to offer wider-scope fixed prices for HS801 than did BAC on VC10MR, or themselves on a DH.121 variant. So, Nimrod. Much the same process later to avoid E-3A for the AEW job.