In our three tier system in the 70s only 30% of children - at grammar school - sat O levels. The tech school children had a choice (O levels/CSEs) and the sec moderns had no choice but to take CSEs. The CSEs (i.e. the exams taken by about 70% of the population) meant nothing, unless you got an A grade which was 'seen' as equivalent to an O level (what grade O level, I don' know).
At 11, a child's fate was sealed by the type of exam they were to take at 16.
In the 70s, unlike now, many jobs required neither qualifications nor fancy application forms. You turned up at a shoe shop willing to work, you got the job. No mock sales to test your selling technique; no need to list your GCSE grades.
Life is different now, and, even though I live in a grammar school area, many children from the sec modern schools can aspire to go to university, even to Oxbridge. When I was doing my 11+ that door was already slamming in most children's faces due to the elitist exam system.
Is this just fluff to please the DM readers - ex-journo that he is?