sixth form recruitment is a two way street, you want a sixth form place, AND sixth forms want to be full. In my experience, you apply and get an offer, then turn up on enrolment day, whether you have met the offer or not. If you have met the offer, you get a place, if not, then you might still get the place, or a similar one.
Then after the enrolment for offer holders, enrolment is opened up for non offer holders, normally on a different day.
People are not generally turned away from sixth form education. WHat might happen is they are turned away from the course that was their first choice, and offered something with lower entrance requirements, with the option of advancing later
Retaking GCSEs is also often an option as a first step. All this depends on the institutions approached, some are selective, some are not. But everyone should, legally get a sixth form option of one type or another. SOme local councils are less proactive about this than others