"I think that private primary is a tremenous investment if you have a really dim child."
OK, aside from the dubious "dim" label this is not necessarily the case. Just as there are good, OK, and not-so-good state primaries, there are good, OK and not-so-good private primaries too.
There are a couple of private primaries near me where I would definitely not send a child, especially if they took a bit more time to learn things than some.
Some private primaries do things like:
Create an "exam factory" where all they really focus on is getting a child through 11+ or independent secondary exams;
Refuse to differentiate work within a class, arguing that is much better for them all to learn together because the slower ones keep up with the others (which they don't - can leave a child with very poor self-confidence and believing that the just "can't do" maths or whatever).
Put pressure on parents (and sometimes children) to do independent entry exams ( because it boosts their statistics?) even when they have decided not too.
Have really old fashioned resources (especially reading books - Kipper et al would be quite modern compared with some.) This wouldn't matter if they were good resources but a lot of the reading books are soooo boring.
Do nearly all their teaching as "chalk and talk" from the front. Not good when children can learn so much better by using "hands on" equipment, discussing and exploring for themselves etc
Can have unqualified teachers, which may not matter if they are good, but will matter a lot if they aren't and only base their teaching on what they experienced at school 100 (or thereabouts) years ago.
Can have Headteachers who are not qualified teachers and may never have actually taught themselves - again this can be OK, but it does sometimes mean they just run the school as a business without a good enough awareness of what teaching and learning is about.
I could go on... but I won't!
Of course there are some great independent primaries too, just as there are some great state ones. The important thing is choosing a school that will help your child learn well and grow and develop as a person - whether that is state or private.