This is my experience which might help, one of my children had a headteacher review as he did not get the score expected. The HT writes a short report and then submits the child's books to a panel who decide if he should be deemed suitable for a GS place. I have a friend whose child spectacularly bombed the test last year but "passed" the HT review so when they got the Kent test results there were deemed GS suitable, even though they had a very low score. There is a section on this site here about it http://www.elevenplusexams.co.uk/school ... nt-11-plus
You can then, as we did, go to appeal on grounds of non-qualification even if you "fail" the HT appeal so I think that option is open to all, although you can't go to appeal until after allocation.
Hope that helps and happy to be corrected if anyone has more inside knowledge, this is just my experience.