The other "unless" for the Min Ht Rule is "..otherwise authorised by the competent authority". The ANO defines "competent authority" (in the UK) as the CAA.
One presumes that published instrument approaches are authorised by the competent authority, ie the CAA.
If you read the Rules of the Air carefully, you'll see that this exemption is for the aircraft, not for the route to be flown.
There are for me three compelling arguments that illustrate that unpublished approaches, while they may undoubtedly be unwise in some circumstances, are not illegal:
1) There is as FFF says no notification of published approaches for the purposes of rule 29. There is therefore no legal distinction between published and unpublished approaches.
2) There are no published departures for airports in uncontrolled airspace in the UK. Thus if exemption (a) to rule 29 must be applied for departure, it seems reasonable to apply it for landing.
3) The CAA has issued an
RIA that implicitly suggests that unpublished IAPs are not illegal.