Interestingly the Apache requires to operate on QNH for targetting but I still prefer to operate on QFE for landing and take-off.
One other thing that has been mentioned is the added requirement to change an altimeter pressure setting as a point of distraction; this is a slight red herring as you have to change from RPS to another setting anyway whether it be QFE or QNH.
As intimated earlier there is much more 'heads in' checking of altitudes to be flown under the QNH system as opposed to the ease of QFE practices. Again the terrain clearance issue is negated by procedure.
On deployed operations, most of the time, UK forces will be operating with UK military air traffic so issues are negated. That said it is not a problem to operate on QNH with other nationalities air traffic, just more heads in at the plates and detracting from lookout, IMHO.
The point about a common QFE at Kinloss/Lossie is another red herring as either way it would be a common QFE or QNH and height errors at the non manster airfield would still be evident for landing and take-off. The common QFE is to facilitate seperation between traffic on different frequencies in close proximity to each other. Incidentally the height errors are negligible as they are both near sea level.
It worries me greatly that anyone one would advocate disregarding either the MATZ in toto or indeed ATC request/direction within a MATZ. A height allocated by mutual consent is far better than to insist that you will do things at your height and air traffic 'be damned'. Military traffic can be of a vast variety of types and in the event that the airfield in question was dealing with an emergency for example, how would you as an aircraft captain feel if you were instructed to go around with an emergency because another aircraft insisted they were going to erode the seperation criteria required for a safe approach?
As for the below 500 feet rule, would the same question be valid for the Heathrow routes that require you to be not above 800 feet when over what I would assess to be a congested area? Does the same principle apply?
The same rules apply to a military helo that has not been authorised to low fly in any event, the question does not lie entirely in the civil arena. If not authorised to low fly would I comply with the instruction, sure I would if my risk assesment led me to believe that it was safer to comply with the air traffic instruction. This doesn't however answer the underlying question of whether it is legal or not, that I dont profess to know............