You can apply even if you don't get too level 5s but you need strong support from the school your child attends in Y7. You can appeal if the LA say you can't sit the tests - theses appeals have a much higher chance of success.
I know someone (OOC) who missed the deadline for the Bucks 12+ and was told they couldn't sit the exam but would have to wait another year and try for 13+. There does not appear to be any scope for appealing in this situation, though it does seem a bit unfair as the child concerned is not doing well in Y7 at the current school - the standard of his work has plummeted since achieving three good Level 5s last year and parents think a more rigorous academic environment is needed to get him back on course. Unfortunately they didn't find out about 12+ until too late so child must languish for another year and possibly see performance fall even further.