The NHBC Standards are quite clear on the DPC height:
DAMP-PROOF COURSE
5.1 - S9
Moisture from the ground shall be prevented from reaching the inside of the building.
A damp-proof course should be positioned at least 150mm above finished ground or paving level and should link with any ground floor dpm.
The dpc should be of the correct width and fully bedded.
Regarding the Building Regulations, measures suggested in the Approved Documents are as you say
"recommendations, best practice, proposed solutions" in other words design criteria measures that would enable compliance with the actual regulation. I agree that the contents Approved Documents should be mandatory not guidance or optional designs.
Installing a DPC 150mm above ground level would obviously be one method meeting the Prevention of Damp regulation.
Then an injected damp proofing system would also work, as would fitting an impervious barrier to the external wall although seldom done in practice.
A "low" dpc (below 150mm above FGL) could mean that your home does not meet the Building Regulations for "prevention of damp." The NHBC warranty would step in if damp did start as their own Standards have been breached.
In any event, a surveyor worth his salt would almost certainly note the low dpc in his report when you come to sell and your buyer would probably base their offer lower as a result.
The NHBC Standards for suspended ground floor ventilation are:
5.2 D10
(b) ventilation of underfloor voids
A minimum void of not less than 75mm should be provided below the underside of floor slabs and beams.
On shrinkable soil where heave could take place, allowance should be made for the void to accommodate the following movements according to the shrinkage potential of the soil:
high potential - 150mm
medium potential - 100mm
low potential - 50mm.
In all regions except Scotland, all underfloor voids should be ventilated by openings providing not less than 600mm² of open area per metre run of external wall, equivalent to one 225mm x 75mm clay air brick to BS 493 at 2m centres.
In Scotland, all underfloor voids should be ventilated by openings providing not less than 1500mm² of open area per metre run of external wall, equivalent to one 225mm x 150mm air brick at 1.5m centres.
Ventilation openings should be provided on at least two opposite sides. Where this is not possible, effective cross ventilation from opposite sides should be provided by a combination of openings and air ducts.
Where the finished level below the floor is lower than the finished adjoining ground level, appropriate drainage should be provided.
The reason the NHBC use the word "should" rather than "must" is because there are several ways of achieving compliance with their Standards and the Building Regulations and an infinite number of designs. (Basements come to mind.)
If it "should" and "doesn't" it is not complying!
As for past factual outcomes, these are irrelevant as wrong is still wrong and what you have must be addressed.
Regarding the NHBC standards, I am surprised as an architect you do not have access to have a copy.
The Building Regulations are all noted and referenced in the Appendix section for each NHBC Standard requirement.
STATUTORY AND OTHER REQUIREMENTS
5.2 - D2 Design of suspended ground floors shall comply with all relevant statutory and other requirements
A list of statutory references applicable to this Chapter is given in Appendix 5.2-A.[/b]
In my opinion, the NHBC Standards in the most part, exceed the basic requirements of the Building Regulations .
They are written in clear, easy to understand, plain and unambiguous English.
Even so, it would appear that large plc housebuilders such as Countryside, are totally incapable of building their homes to meet the Standards and when they are 'found out', will put in every effort to find justification and excuse to avoid rectification works at all costs.
If the NHBC refuse your warranty claim, you can take the matter to the Financial Ombudsman Service.