DFC says
As for DIY approach procedures.
Yes indeed, the 1000ft within 5nm of the aircraft requirement does not apply when approaching to land. After all at touchdown the separation is 0!. Furthermore, when completing a published approach procedure, the obstacle clearance will be at times less than 1000ftand the area taken into account far smaller than 5nm from the aircraft.
The leg - Rule 5 - actually says "Exemptions from the low flying prohibitions
(a) Landing and taking off
(i) Any aircraft shall be exempt from any low flying prohibition in so far as it is flying
in accordance with normal aviation practice for the purpose of taking off from, landing at or practising approaches to landing at or checking navigational aids or procedures at a Government or licensed aerodrome.
(ii) Any aircraft shall be exempt from the 500 feet rule when landing and taking off
in accordance with normal aviation practice.
I leave it to you to decide whether what DFC says is normal aviation practice.
The straight answer to part of the original question is that for a non-commercial flight outside CAS it is not contrary to any part of the ANO to make an instrument let-down that the pilot creates. The CAA obviously doesn't like this - there are a couple of proposals that are being/have been consulted on
here and
here that make this clear. Although one of them is now quite old and doesn't seem to have led to anything I don't think it has gone away because I spoke with someone from CAA a week or two back and they said it is still under consideration.
As for is letting down in the circumstances that Dude~ first describes, speaking as a controller I would say that even if you stay legal it is unwise! There's a reason that proper IAPs need all that effort.
DFC also says
There is one other fly in the ointment..........the navaid used must have been approved for use as an approach aid. One can not use an enroute VOR/DME as an approach aid.
Not true at the moment. Lots of navaids used to have "Not to be used for instument approach" in their details in the AIP but its not there any more. If it was Rule 40 would cause you a problem because it says "the commander of an aircraft shall not make use of any radio navigation aid without complying with such restrictions and appropriate procedures as may be notified in relation to that aid unless authorised by an air traffic control unit."