True, 60/12 = 5.
In reality, kids in that classroom won’t be getting five minutes or even two minutes on 1-1 teacher time, because classroom teachers spend most of their time teaching to the whole class. And any bits of snatched 1-1 attention that your child does get from the teacher will be poor in quality, because the teacher is simultaneously trying to listen out for the rest of the class, keeping an ear out for any kids who are having problems, whispering, getting off-task and so on. Yes, even in a private school classroom where the most difficult kids have been selected out!
I’ve taught in tiny classes of hand-picked kids with no special needs or behavioral problems, and I still didn’t have time for focused, multiple-minute-long one-on-one tutorials with kids on lesson-related contents in the middle of the lesson.
Paying for a private school for the reason of “I want my child to get individual teaching for maths and English etc.” makes not a lot of sense; it would be a lot more effective (and about ten times cheaper) to just use your state school, work out what specific areas your child would benefit from individual support on, and pay for some tutoring in those areas. Bear in mind that a child in school, by definition, spends a lot of time in classes you might not care about (PE and music, in our case), in classes where they already excel and need no individual attention anyway (art and language arts, in our case), in form time and in break time and so on. Yet with a private school you are paying for those expensive staff:child ratios round the clock, for all of their lessons, whether it makes a difference to them or not.
FWIW, we use a private secondary school, but for other reasons, not small class sizes. If our reasons and circumstances had been different, a state school would have been fine.