I think it varies so much both from school to school, and over time.
I was in the second cohort to take GCSEs, back in 1989! So picked my options in 87. My school only allowed 8 GCSE options, although some of the very bright girls were allowed to take a 9th. (Not me!)
We HAD to pick: English (lang), Maths, a language, a science and a humanity. The other 3 were free rein. So I did English, Maths, French, German, English Lit, Geography, History and Biology.
Fast forward to DS1 picking his options 2 years ago: his school pick 12 options! Which really doesn't give much scope for dropping anything. Their compulsory subjects were English Lang, English Lit, Maths, ICT, a language, and they could only choose EITHER all three sciences, or dual award Science - I'd have hated that as I was awful at science subjects and only did Biology as I absolutely had to pick one - he went for the 3 individual. Students choosing this option then had to pick any 4 other subjects, those going for dual award science could choose any 5 others. There were no "boxes" or subject groups like we had to pick from; any combination was allowed - within the constraints of timetabling, of course. He also takes a 13th subject (Greek) but learns this after school so it's not one of his timetabled classes. His subject list reads: English Lang, English Lit, Maths, ICT, French, History, Geography, Business Studies, Biology, Chemistry, Physics, Latin and Greek!