One characteristic of the People's Republic of Milton Keynes is the vast empty spaces around the city which are not really 'congested areas'. Indeed it would be quite possible to glide clear of the centre of PRMK with an engine failure and make a suitable landing area. It's quite difficult to see where the '1500 ft' rule part of Rule 5 would apply over such a peculiar city IF the 500 ft part WAS being obeyed.
But perhaps they were just checking to make sure the concrete cows hadn't been struck down with concrete foot-and-mouth......