It's not easy!
The AIP and the ANO go into great and careful detail to define the minima (vis and DH/MDH etc) for IAPs, but make no mention of minima for IFR approaches where there are no IAPs.
So the question is "How can you define when it is legal to commence a descent for landing when there is no IAP?" Is it permissible to invent your own? Perhaps we should ask the CAA.
There is a table of minima for non-precision aids which sets absolute minima (for an IR pilot) starting at 400 feet and going up from there. (For precision aids, they are lower).
There are also minima for circling approaches when descending off an IAP to land on a runway without an AIP. It is also prohibited to use any other than an approved IAP when making such an approach. But none of that seems to apply to aerodromes without approved IAPs. Should we define that as "permission", or "automatic exclusion"?
ANO Article 40 defines 1000 feet as the minimum height to continue an approach on a notified IAP if the RVR is outside minima. It doesn't mention non-notified IAPs at all.
My conclusion is that if I had an accident using an unofficial IAP, and descending below 1000 feet, the CAA would find a valid reason to prosecute. I don't think the exception in ENR 1.3.1.2.1 would be a very good defence.
I think I'll stick to the method I was taught - divert to somewhere with an IAP. If necessary, fly VFR below cloud to my destination (remembering VFR minima in the airspace I'm using).
Where's bookworm when we need him?