Even the grammar schools have vacancies for sixth form - I know of a few bright kids who've been to a comprehensive for years 7-11, got a boatload of GCSE's then entered the grammar school for A-levels.
In terms of entrance requirements, some grammar schools have exams (in the A-level subject choices), and some make offers based on interview/application form only. The offers are conditional on GCSE results, obviously the student has to do very well at GCSE.
Failing the 11+ doesn't mean you'll never get to grammar school.