I was about to say that only maths is a good idea to do early. A bit identifying, but anyway I then recalled that actually I did all of them early because I skipped a year in school, so maths was 2 years early. Moving ahead in school was challenging socially at ages 10-13 (while I had friends, some other girls were mean), but beyond that it was fine and exams were never too difficult, so I'm not sure about maturity for certain subjects. While I could say that taking exams a year early doesn't affect grades, what's different is that I'd missed second grade (age 5-6) so I'd had the full number of secondary school years and that experience might have actually mattered when it came to maturity of response in essays etc. What's a shame is that these things are so rigidly timetabled. Having been through 4 A-levels, a degree, and a career with times of extreme stress, conflicting demands and long working hours - I still recall GCSEs as being excessive pressure when exams in 3 subjects turned up over 2 days. I don't know why it's set up like that when it genuinely doesn't reflect real world conditions. Particularly because kids will not all have a level playing field because they have different exam schedules. Coursework can teach kids about balancing conflicting demands much more clearly than exams where there is never "enough" revision. Sure, some can cheat, but just proportionalise the level of coursework against exams to balance that risk.
I think taking a range of exams from November to July would be better all round. There would always be low-level exam pressure, but kids could be taught how to deal with ongoing stress rather than learning how to select tasks from an immense potential workload while sitting panicking in their bedrooms alone late at night May/June. Splitting multiple exams per subject through the period would focus those who study badly earlier, earlier exams could be lighter in coverage. Re-takes could also be seamlessly fitted in for those kids who were unwell or who had problems during the period, without anyone needing to note it as re-takes because all results would be Year X. I think students would overall learn more and find it less nerve-wracking. Exam bodies could employ professional writers, markers and quality checkers year-round to professionalise these roles; spreading all their own exams through the year would reduce the strain of exam writing, issue, collection, marking, risks of theft etc. Schools could adjust timetables to have focused revision classes through the year depending on their exam timetables; Maths (foundation), French (listening & understanding), English Kit 1, Geog... French (written), Maths (algebra) etc. I'm sure someone will have a good reason why this couldn't work, but that's my idea.