EF2020 you failed observer aptitude tests. You did NOT fail navigator atitude tests. Let OASC decide whether you are fit or not.
Then remember they are recruiting officers before aircrew. You will be an officer all of the time and aircrew only some of the time.
If you apply to be pilot or bust then they will happily assess you as pilot or bust. If you then failed pilot aptitude they would not offer you nav.
Not sure what is now involved in nav apptitude as they have fially stopped recuriting for the Lancaster force and there is precious little difference betwixt back and front of a 2-seater and lots of the ME are also 2-seaters!