Ideally exams should be a fair test for everyone taking them. It should be suitably difficult so that weaker candidates are actually able to get started with questions, but also so even the very best candidates will struggle to get most of them done.
Given that there are a significant amount of people able to breeze through A-level exams with 100% questions whether the exams are actually holding up to their purpose which is to distinguish between people. Personally, I don't think they are.
Admittedly the "significant amount" is very much a minority, but there are enough people achieving it to mean that differentiating between the top 0.5% is not possible.