why would they use gCSE results for a-levels?
Why not? How can it not be a relevant indicator of ability/aptitude?
Say, someone working at grade 7 in class, homework, mocks, etc., for Maths, gets a grade 9 after buckling down for revision etc. Now working at grade A during lower sixth and mock A levels, needing A* for Uni entry requirement. Surely the jump from 7 to 9 at GCSE is a very good indicator of not only effect of revision, but also ability etc.
Of course, where the course is very different or where a subject wasn't taken at GCSE it's not relevant, but when an A level course in say Maths or Science is basically the same but to a higher level, then the GCSE grade achieved must be relevant.