GCSEs are in the category of the lowest formal qualifications in the U.K. Employers and higher education institutions almost always require at least Maths and English, and often 3 additional subjects, at grade C or above. Language GCSEs are not incredibly demanding, at least in my experience - I got an A and could only have a very basic conversation in the language. Mia could probably do well in a Spanish GCSE even now, though she’d probably have to study a bit of the set curriculum to know which topics could come up
Anyway, she could do a Spanish GCSE but as I said that alone might not be much use as many places require 5 GCSEs including English and Maths. When she’s achieved that, she would then have to do 3-4 A levels in order to access higher education. TESOL courses require these qualifications. A very good knowledge of English is also necessary, so she would definitely need an English GCSE and an English A level would be advantageous if not essential.
Of course there are stories of people succeeding with no qualifications but there are many, many more of people succeeding with them. They open up so many opportunities and it’s sad to see Mia being brainwashed into rejecting them. GCSE exams are generally taken age 15/16 so Mia is only just starting to fall behind but I don’t see anything changing unfortunately