I hold no brief for Mrs Thatcher's and John Major's governments, but there was no National Curriculum in England until the early 1990s. It was the teaching profession, probably led by fashions in teacher training, that decided not to bother teaching English grammar, and presumably the exam boards followed suit.
Before the National Curriculum came in, with SATs introduced to see how well children had learned what was in it, league tables of SATs results published to give schools a spur to get better results, Ofsted going in to see what was actually going on in schools (all new features of education in the early 90s), it was entirely up to teachers to decide what to teach and how to teach it. There was no performance management in schools and it was rare for teachers to be observed at work, never mind assessed and given targets for improvement.
Obviously in secondary schools pupils were prepared to take external exams (O level, CSE, GCSE and A levels) so they had to follow the exam syllabus at that point, but prior to that many teachers did their own thing. In some primary schools headteachers insisted on a common approach and obviously teachers usually wanted to work from the reading and maths schemes in use in the school because those were the textbooks they had available to hand out to their classes. There was no requirement to work through them at a given pace, though.
My Mum was a pimary school teacher until the mid 1980s and she used to mention a colleague who always taught what we would now call year 3, i.e. first year of Juniors, aged 7-8. There was a parallel class of the same age. They were not streamed by ability, so both classes were mixed ability, and in theory should have moved up to year 4 at the same standard. They didn't. It was an accepted thing that whoever inherited Audrey's class would have a much harder year than the teacher of the parallel class because she would have a lot to do to help them catch up. Everybody knew this, probably including the parents, and nothing was done about it. It was just one of those things.