I don''t know why some people are so sniffy on this thread.
I'd rather have an Oxbridge grad (with a decent 2:1 min) teach my kids than a lot of teachers because I know, if they are a clever reflexive type of a person, they know a lot about exam technique as they actually have the type of grades I'd like my children to get. Most teachers don't, some of my students with 2:2's or even low 2:1's who aren't that brilliant want to be teachers.
I have been a tutor, it was relatively easy, some content delivery plus exam technique plus practice. You don't need a PGCE, just be extremely good at exams/assessment yourself and able to read the syllabus, for a limited number of subjects/boards.
I'm sure having the chief examiner would be even better, but there aren't so many of those to go around.
If the OP gets results, and has a list of satisfied clients, then I would go with that- because if they have done that for others, they could do that for your children. Someone may be a great overall class teacher but not particularly specialized in moving children from an A to an A* (or 7 to a 9).