Interesting that you need extra training to reduce the DH from 200ft to 150ft. If my brain is working today the RAF Seaking had a DH of 150ft for an ILS and PAR - this was due to the calculation of DH for various a/c which included an Aircraft Approach limitation (AAL) which was used in conjunction with the Obstacle clearance limit (OCL). This produced a DH for the Seaking of 150ft - we had no extra training for this - you just flew to that limit.
Crab may be able to confirm if this is still the case - the ILS was also manually flown as we had no coupler for approaches to airfields.
HF, as HC mentions above, we are limited to 200' on an ILS due to the system limitations not the helis. We can still subtract 50' from a FW precision approach minima but not below 200' for ILS and 150' for PAR.
Unfortunately, after many many, years of safe operation of the SK, someone allowed a piece of work by a Navy TP to surface and we now have a variable HTA between 30' and 50' to add on
Oh and the same genius imposed a 100 kt GS limit on ILS as well - not very helpful when you have poorly people in the back and you are in a hurry!
I believe that the S61s in Ireland (which had the SN 500 series autopilot like the SK 3A) were allowed, in extremis, to use the trans down SAR modes to runway which would get you to 40' fully coupled.