Resit alevels

Does anyone know any schools in north west London that offers resit opportunities for external students?

The Joint Council for Qualifications (which the umbrella body for all the major exam boards) has a search tool here.

Note that the list it provides are exam centres, and includes several with NW postcodes. These are places where you can sit your exams as a "private candidate". This is not a list of placed which will teach you.