Pretty sure it was a minimum of a B in the subjects you wanted to study (based on the school's idea you'd probably go down a grade at A level), and I think there was the 5 A*-C grade stipulation as well, but I don't think they were too fussy on it.
But tbh by year 11, pretty much all the people who wanted to go to sixth form were on track for way more than the 5 A*s to Cs, and those who didn't had either decided long ago they wanted to go to a sixth form college because they didn't like the school/its lack of options, or were already doing some form of college course (in our school, you could go to college to do mechanics, beauty, building etc in lieu of doing 2 or 3 GCSE subjects).