LondonGirl is not allowed to know which applicant is which. London Girl sets them an extensive grammar test and an essay to write. She marks their papers. The bloke with the credentials scores 10% whereas our bus driver gets 99%. Big problem now. Do we give the job to the former anyway because he has the credentials?

I have many very well spoken friends who write beautifully... However ask them to explain the difference between present perfect and past simple and why there is a difference and they couldn't.

Which is why for teaching, the one who has completed training and knows how to answer the questions their students will have is better suited than an articulate bus driver.

Quite often, advanced level non-native speakers are more away of the mechanics of a language than native speakers. I know a lot of my FCE+ students speak better English than a lot of English people I know!