Dare I suggest that the main point has been lost over the last few posts ? :)
I don't see where the timing of taking exams comes in.
The changes to GCSE will prevent students taking and re-taking modules at GCSE or more specifically A-level in order to 'spread the load' of learning OR to slowly improve upon a grade before a final grade is awarded.
This is something that many universities have had reservations about for some time.
This is totally different from someone applying to university as a mature student, who has gone back to college post-18 to gain more qualifications.
As for choices at 14 being binding- well they are in the majority of cases because options in years 9 and above impact on GCSE and A-level choices. If someone leaves the system then studies elsewhere as a mature student of course they can study other subjects, but for the average 16-18 yr old, options at 14 are binding whilst they are in full time compulsory education.