The datum point of Ongar was used because anywhere else (except Chesham) would have required some points to be negative.
The Northern City Line still used miles from Moorgate in 1975 (see the Official Inquiry into the crash). By 1972 the NCL was not directly connected to the rest of the system - the Highgate route had been closed in 1971 so stock transfers had to run via Kings Cross and the "Widened Lines". Neither Brill nor Verney Junction were part of the Underground in 1972.