Does anyone know, if a person applying for whatever music course already has a diploma in teaching/performance or whatever, would it still matter what kind of results they got in a levels/leaving certificate or whatever? Could they just apply with that instead of school exams?

Entry requirements, as far as I know, are done on a level grades or the equivalent. A diploma would just be an added bonus, I am sure

so the only way I could avoid that would be to wait until I can apply as a mature student, would it? Or would that not make any difference.........

Officially, yes, they do entry requirements based on the audition (mainly) and then require a minimum set of A-level grades too. If you read their websites, however, it says that the A-level requirement can be waived in exceptional circumstances. If you amaze them at the audition then they will let you in regardless of your A-levels; however, bearing in mind the standard of even the people who are rejected one would have to do something pretty special to stand out and amaze them.