These threads just run and run. Luckily, there isn't much data on what happens in bodged DIY IAPs.
Graham Hill was one, apparently. That one went badly wrong in the courts; I gather from an old timer who was familiar with the case that his IMCR had expired. Is the report online anywhere? That would put it clearly "out". But in those days a lot of planes, including airliners, were flying into terrain. Today, there is no excuse because even the cheapest GPS tells you where you are exactly, some 99.99% of the time.
In the absence of licensing/airworthiness issues, insurance will pay out in a case of negligence. That's covered by insurance. Not much use to the pilot because a bodged IAP (DIY or official) is likely to result in a CFIT and they are usually fatal. But it could affect whether passenger claims exhaust the 3P cover and hit the pilot's estate.
Not always fatal though; I know of one where he survived and there wasn't any legal drama. He did a GPS based IAP with no supporting position fix, among terrain. But what got him was not the descent, it was the lack of a missed approach procedure.
So, what seems to matter in these things is not CAA action (especially as the pilot is likely to be dead) but insurance issues. And insurance does cover negligence - up to the numerical limit of the cover.
All in all, not enough to go on and just as well!
IFR, one has to stay at the MSA unless landing and then how low can one go? I would do 700ft AGL over fairly flat country, with a position fix via two independent methods, plus current QNH and altimeter verified (for gross error) against an IFR GPS. Or 500ft over the sea. Otherwise, a low level letdown (radar based, obviously). The trouble is that a low level letdown is that they will take you down only to 1000ft AGL anyway. All that does is that it supposedly eliminates obstacles that you didn't spot on the chart, but you are trusting the ATCO completely for terrain clearance because he is giving you vectors and expects you to fly them.
More important, IMV, is to establish the cloudbase in the area. If a nearby unit gives it as OVC003 there is no point in trying anything. Perhaps a really experienced ex-mil low flyer could do OVC003 over water, to land on a runway which is right on the coast. If the whole area is around BKN010 and it is fairly flat, that should be perfectly safe for a DIY descent using a double position fix, and this is over the sea also.
Just my 2p. You have a point but the CAA are just AN authority on what is safe, not the final authority. Given no FDR/CVR one usually cannot establish what really happened anyway. They couldn't prosecute a dead pilot; they could offer an opinion in an insurance argument. A lot of what the CAA have written (for GA consumption) is bunk, especially on slightly modern things like GPS. There is so much stuff that is uselessly vague (or implies that it overrides manufacturer's guidelines) that a lawyer would have a field day with it.