This thing about superselectives is moving away from the topic of the thread, but in terms of that, I think some people perhaps aren't understanding the idea of superselective vs selective. Any school which has an entrance exam and more applicants than places could be described as selective. However, the level of selectivity varies - superselective is usually used as a term for state Grammars which admit purely by score without reference to geographical location, and hence which draw from a huge geographical area and attract vast numbers if applicants and only a tiny proportion receiving offers - hence superselective. It's difficult to apply the same term to independents because none of them have geographical catchments, but I think the term is being used on this thread to refer to those which are most difficult to get into, most competitive and demand the highest academic standards to gain entry. On this basis, of course BC is selective as it has an entrance exam - no one is saying it has a comprehensive intake. However, there are other schools which might be harder to get into and this is where people talk of top and second tier schools, although it's a matter of opinion about where lots of schools fit within these pecking orders.
I'd expect we can all agree that making comparisons between GCSEs and IGCSEs is complicated by the fact that GCSEs moved to the 1-9 grading earlier - the short period of time where one was running a reformed system and one wasn't might well mean there wasn't parity. Even once they are both reformed and using the same grading system there might still be debate about one being easier than another, in the same way those discussions always exist about different exam boards. People have different views and these change over time. Unless everyone has to sit exams orivided by the same examining board these debates will always continue and we will have to accept that exact parity won't exist in the same way it will never totally exist between subjects either, despite efforts to improve it. And we will have to accept that schools of all types will always be looking to perhaps prepare their students best for the next phase of education, or to take the exams which will look best on paper for their students entering the next phase of education or for themselves in league tables. The whole system incentivises such behaviour doesn't it.
It always strikes me as a bit odd that those who say league tables are useless for a variety of reasons are often those who then get most het up about the impact of these seeming differences in parity and want adjustments made for them. Isn't the whole problem that there is subjectivity and even any adjustments wouldn't meet with universal acceptance and agreement. Again, until the unlikely event of everyone taking exactly the same exam, there can never be total parity in grades,.