Have just made the decision to send dd2 to private school rather than the primary dd1 and ds are at.
Here's why:
The classes are smaller -- 20 rather than 30+, and each class has a teacher and an assistant;
The range of ability and behaviour is more manageable, so less class time wasted on dealing with behaviour and more time supporting children to do work which actually matches their level of ability;
A wider curriculum. My older kids' primary school achieves its (good) league table position by doing very little other than literacy and numeracy;
Better extra-curricular activities. My older children's school has quite a good range for a state school, on paper, but in practice most of it doesn't really happen, or happens for a bit and then fizzles out. Music lessons ended up being about 5 mins long, so children barely had time to get their instrument out;
Better teachers. Controversial, this, as there have been one or two outstandingly good teachers in the primary school, but also a lot of short-term twenty-something Ozzies, who don't have the long-term view or the experience to do their best for the children;
Freedom from the National Curriculum, which inhibits teachers' freedom to think beyond the obvious and do more creative and imaginative learning with the children. For example the private school has beautiful displays on all the classroom walls (and no, it wasn't an open day when we visited, just an ordinary one) whereas the walls in the older children's school are mainly bare, apart from the odd poster;
A mindset that doesn't reduce all teaching and learning to Level 4 at age 11, and encourages children to stretch and challenge themselves.
I could go on. It's not a move we've been looking to make, we're not particularly pushy parents, but I do feel that the school (and by extension we) have failed my oldest child in particular. Also, I no longer have the energy to do for the third time all the extra input that you have to provide if you want your state-school educated child to be anywhere near the level of a private school child by the time they finish primary.