Indeed, the "comps" often weren't as comprehensive as they should have been. Similar happened to me. You had to choose O levels by selecting one subject out of each "block". Sometimes, some subjects were only in one "block". I wanted to take computing and Latin but they were in the same "block" so I had to choose one or the other. Then, because I was in top set for Maths, I had to take RE (all the Maths top set had to do RE as an extra O level). I remember class mates complaining that they couldn't take both Art and Woodwork because they were in the same block. Yet, you could take all 3 sciences and 2 languages alongside Maths & English.
Most schools still put optional subjects in blocks, so that lessons (and teachers) can be timetabled efficiently. My school's policy was to have one block exclusively modern foreign languages, effectively forcing everyone to study one. (Getting out of that option block was a rare thing, and required your parents to come in and make their case to the headmaster, iirc.)
In my cohort, those that didn't select IT in the "technologies" block all studied a short-course in IT, as it was considered important enough that everyone should do it.
The grammar school I went to in the 1970s didn't offer much choice at all at O level. Everyone did Maths, Physics, Chemistry, English, French & Latin. Then one more out of German, Ancient Greek, Russian, Geography or History.
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I'd have thought any school today would have to offer a wider choice of subjects for GCSE than I had for O level back in the 1970s
At my comprehensive, two decades ago, everyone would be studying for the following GCSEs:
English Language and Welsh Language (1st or 2nd language course, as appropriate)
Maths, Science (double award)
Four optional subjects
Those options would include one technology, one MFL, and two other subjects (e.g. art, geography, history, additional MFL, business studies, religious studies, PE)
Those in the top language sets would also be entered for English/Welsh Literature exams. So most students would be studying for 9-11 GCSEs.
These days (with no formal KS3/Year 9 tests) many schools in Wales are starting GCSE material early to put pupils in for exams in year 10. Top sets in Science may study triple award (so effectively a GCSE for each science) and study Further Maths in addition to GCSE Maths. The Further Maths qualification is a very useful bridge to A-level: having taught it a few years back I know I would have benefited from getting an early look at all the new and unusual concepts I'd be doing in A-level Maths, many of which didn't really sink in until University.