In my day, the five years after the 11-plus at the end of juniors (if you were in an an area that hadn't abolished it, as most had; if you were the top few went to Grammar School, everyone else went to comprehensives. Elsewhere, everyone went to comps).
Either way, you then had five years till O-Levels (or GCE, the 'stupid' exam. Not fair, but that was the image). Until 1988, when the new GCSE replaced both.
Unless, of course, you were at a Grammar like mine, where we didn't have a third form, and did our O-Levels in the fifth, but a year early, followed by a three year sixth form (lower, middle, upper) rather than the usual two (don't worry if this makes no sense).
It all got changed in 88, and then again since, but the basic idea of a separate two years from 16-18 holds. Even though it's a bit redundant now the minimum school leaving age has been raised from 16 to 18...