For context, I actually support the use of unconventionally qualified teachers.
I am a second in department at a very respected international school. My first degree isn't in the subject I teach, although I do have a PGCE & 20 years experience.
One of the best teachers in my department has no formal teaching qualifications. She fell into teaching 30 years ago after accompanying her teacher dh to Forn Parts.
Should she be eligible to teach on the UK (she's eccentric but not totally bonkers, so unlikely she'd want to tbh). Absolutely. There should be a fast track mechanism in place to UK QTS.
On the job schemes like Teach First? Fine.
Where I part company with User14 is that by her own account the school in which she has a somewhat amorphous & debateable role is quite dreadful. TAs are incompetent & not managed or supported. Exam administration is blood curdlingly incompetent to the point of jeopardising the legitimacy of results. Apparently, students telling teaching staff to fuck off & leave them to play on their phones is both endemic & unchallenged.
At this point, when user14 tells us that unqualified teaching staff are just as good qualified staff, I'd argue that she has failed to prove the case.