@Elbowedout I suppose it is harder for them to gauge how a student might manage a heavy workload if they have done GCSEs in several batches rather than all at once.
Iswym but if you're taking 3 or 4 A-levels simultaneously then that's certainly demonstrating ability to handle the workload more recently, and in a way which is more relevant to higher study than GCSEs. Most applicants will receive conditional offers, so their place is dependent on them handling the A-levels. It's interesting to hear what your contacts said - all I can say is that I've spoken to a number of people in university admissions and at outreach events, specifically for Russell Group and Oxbridge, and have been reviewing official statements on this, and there has been a strong message that GCSE timings just aren't that important. However, it's always possible that some people in admissions will take a different view, and for that reason I think it's a good idea to have some explanation of why exams were taken this way, ideally in an academic reference.
Back in the days of modular GCSEs where you could continue retaking one module to improve your grade, I can see why one might look askance at a strange GCSE profile. If it looked like a candidate was retaking multiple subjects to bump their grades up, you might want to know why. But them having finished by sitting say 5 or 6 GCSEs as opposed to 10 isn't going to make much difference if they are taking their A-levels together.
I agree that the declining popularity of AS - levels has made GCSEs more important. It is still possible for candidates to enter AS levels and some persuade their schools to do this, or enter as private candidates, if they feel they have been on an upward trajectory since GCSEs.
There's a really interesting document from LSE which explains how they compare things like GCSEs, A-levels, applicant background etc. Can dig out if people are interested.