I think overall it is easier to do well at GCSE due to the coursework, one of mine wasn't particularly strong at Eng Lang but managed to get an A due to getting help with coursework, which brought the grade up significantly.
There isn't any coursework in English GCSEs any more (there is in Welsh ones, and Northern Irish schools can choose).
I doubt that admissions tutors are allowed to discriminate betwen an A grade achieved at an English/ Welsh/ private school.
We are allowed to (I had discussions with Qualifications Wales about this) but it would cause so much controversy that it would be likely to have a negative impact on our applications. I think it's inevitable, however, that an English student will take a university to court for requiring her/him to achieve the same grades as a Welsh student when the assessments are so different and the Welsh qualifications can be argued to be "easier" on the basis that they are less reliant on end-of-course exams.
The other point I think is relevant here is that there is a formal mechanism by which Ofqual in England, QW in Wales and CCEA in Northern Ireland work to ensure equivalence of standards, and CCEA introduced a C grade this year to avoid lack of alignment between grades in Northern Ireland and England (i.e. C=4, C=5, B=6, A=7). Cambridge Assessment, which runs IGCSEs, is not part of this as Ofqual no longer accredits IGCSEs.