DD went to a desirable state primary. It would have been a huge hassle and much further traveling to get her to a private primary. (We knew one local person with a child in private primary, mother spent hours each day in her car.)
Although her primary was wonderful in many ways, it was virtually impossible to get any understanding of how she was doing academically, even when explicitly asking teachers in face-to-face meetings. (I still have my primary school report cards, from the 1970's, from a state school in another country, where we got detailed grades, and a teachers comment, for each of several subjects, at the end of every term. And from about year 3 onwards had to sit formal exams, day after day for a week or two, and actually pass these, to be allowed to proceed to the next year. The British system feels like it's supposed to work by osmosis, provided you ensure a child is regularly in a classroom until age 11, no need for anyone to check how or whether they are learning anything.)
When DW signed up DD to a tutor, who asked her to write an essay, she apparently drew some pictures. Ten to twenty thousand pounds of tutoring later, she scraped into the only private secondary school that would have her, after being wait-listed.
Oddly, although (as far as we can deduce) she worked her way up from below-average to average across her primary school career, her first report card from secondary is mostly B's with a sprinkling of A's. Maybe I don't understand British grades, but to me that sound quite good, for someone who's supposedly barely academic enough to be there.
Another new experience since she started her private secondary, is her insisting on her doing her homework, so she can get the grades she wants... (She's never voluntarily done school work before.)