Errm no it wasn't. I sat both GCSE and CSE exams. The 11+ exam saw children qualify either for the local grammar school or secondary modern. Grammar school pupils invariably sat GCSE but some who were not quite as proficient in a subject were allowed to take the CSE version. Secondary Modern pupils usually sat CSE exams but the ones more capable in a particular subject were permitted to sit the GCSE version. As I said before a grade 1 CSE pass was usually accepted as being the equivalent to a grade 5 GCSE pass.
Trust me on this one, unfortunately as my age will indicate "I was there !"
Don't mean this in a rude way, but you're too old to have sat GCSE's in your school years...
They came in in 1988 - General Certificate of Secondary Education - Wikipedia
Were you held back for a decade or so?