The Royal Navy would specify a mission, not demand a jet

Would a Royal Navy-controlled Shackleton replacement insist on a British jet aircraft?

Probably not; the Navy would more likely specify range, payload, and reliability, then assess what industry offered.

Once the counterfactual assigns maritime patrol to the Royal Navy, the service would likely publish a performance requirement instead of preselecting a jet solution. Long range and a heavy useful load would dominate, with industry invited to propose the most reliable aircraft. This requirement-led approach also marks the point where stacked alternate assumptions make a precise prediction unreliable.