Good sixth forms in north London? Watch
I'm going to quote in Tank Girl now so she can move your thread to the right place if it's needed.
Latymer is a really good school and has good A-level results, if you have the grades for it, i think you need 6 As to get in and at least As for the subjects you want to take, Woodhouse is also good if you like independent study and they accept A-B grades and are lenient in that, but it really depends on your preferences and the subjects you want to take, there's also laSWAP, mill hill, qegs, owens, ashmole and fortismere
However I would advise to make a decision quickly as by now most sixth form colleges are full and you'll also need to catch up fast on all the classes you've missed!