The skills of manoeuvring and navigating a helicopter at very low level to avoid detection have to be taught somewhere and Salisbury Plain is too open and featureless for that. Hence the use of LFA1 down to ground level (there is no rule 5 in mil regs but they will work to a Minimum Separation Criteria, it used to be 5m but may well be different now).
It is likely that the gamekeeper approached from the starboard side of the aircraft so the QHI was unsighted until the student spotted him.
Middle Wallop used to spend a lot of dosh entertaining all the local farmers to say thank you for allowing the continued use of their property, I suspect this is still the case.