The only good reason I can think of is that the UK had a very tight focus on subjects: at GCSE you study 10+ things, at A(s) level 3-4, at university 1.5. You can't really use GCSE results for a UK degree entry because 80% of those subjects are irrelevant for this degree because (unlike EU\North America) a (say) physics degree has zero credits for English lit or history or even chemistry. A Level results are too late. So that puts everything on AS results (and I think the Tories got rid of the actual AS exams a few years back?).

I am a big "final exams not coursework please" guy. But they'd be better having end of term exams for the 6 terms in A Levels and using the results from the first 5 for most of admissions IMHO.

Yes, the current government reduced or got rid of much coursework and stopped As levels contributing to an A level.

In the previous system, there was more coursework in some subjects (almost always still <50%), thus recognising the abilities of those who don't do so well in exams, and still recognising the abilities of those who are better at exams than coursework. Exams were split over two years, with most of the material studied in year 1 examined in one go in year 1, and material studied in year 2 (building on year 1 material )examined in one go in year 2.

Now there is an almost total reliance on exams, all in one go at the end of the second year.

Hence (partly) the current storm.