Crab,
With the advent of GPS, FMS systems, Autopilots with four axis capability, RadAlts, TAWS, GPWS.....and FLIR....can you with your SAR experience....envision an IAP for that landing site that if properly surveyed, flight checked, and re-surveyed monthly for changes in the way of obstructions to include checking with the local authority to see if there. have been any Building Permits filed for or approved within the surveyed flight path?
What Minimums would you think workable and safe?
Yes, of course you could create a workable instrument approach to the LS - it has the advantage of being on the top of a hill so the obstacle planes would be fairly benign.
However, you still have to revert to visual flight once you get to your minima since you don't have all the advantages of airfield lighting to help you identify the LS in the dark.
With all the caveats you suggest in place, 150' minima above the elevation of the LS would seem reasonable but probably at a low speed of circa 60 kts to enable quick transition from instrument descent into visual approach and landing.
The problem with this flight wasn't the aircraft capability though - it was poor decision making and self-imposed pressure (as we see all too often) - clearly the suggestion to go straight to Wellesbourne would have been the adult decision.
I note they were both ex-Mil and ex-Sea King but don't know which Service they had been in - that may have had a bearing on their decision-making.