The IMC minima are a bit confusing even to many instructors who teach it.
The 500ft precision and 600ft non-precision limits are absolute. The IMC rated pilot when calculating the appropriate minima for the approach can not use a DH of less than 500ft or an MDH of less than 600ft. The minimum visibility in all cases (take-off as well as precision, non-precision and visual approaches) is 1800m.
As well as causing confusion among pilots along with making the calculation of DH/MDH more complicated, the CAA have it seems allowed IMC rated pilots to use lower minima in the more demanding situation.
To explain this - Airfield X is in open level country and the NDB/DME approach has a published MDH of 300ft. The IMC adds 200ft to get 500ft but must add on a further 100ft to get the absolute minima of 600ft - very safe indeed.
However, Airfield Y is located in mountainous terrain with many obstacles and a tricky NDB only approach with an MDH of 800ft. Since this is equal to the absolute minima of 600ft, the IMC pilot can either add 200ft as recomended
OR use the IR minima of 600ft.
More dangerous however is the fact that someone may try in error to use 600ft!!
Far far better for the CAA to simply state that IMC rated pilots
must add 300ft to the IR minima. Then they can never use the IR minima.
The CAA has also made no allowance for PPLs flying faster aircraft than cat A.
A further gotya is the rule which says an IMC holder can not take-off or land when the visibility is less than 1800m. The VFR minimum visibility in class G for an IMC holder is 1500m. Thus, the IMC holder can fly VFR enroute through 1000 miles of class G in 1500m visibility but can not land at the destination until the visibility improves to 1800m.
Regards,
DFC