We did the move at Year 3.
It wasn't that the primary was awful....just a bit disappointing. Child was a top of his class, but only 2 others got L3s in Writing and actually, they have both left now, so I wouldn't really want DS to be the only one working at his level. I know teachers can differentiate the work but it is good to have others working at a similar level.
Where we are now is better. There are more working at a similar level and they are in sets of 12 for things like Maths. They use more traditional approaches in maths from early on which I like and are big on traditional comprehension exercises etc. There are also more opportunities to do different things, so for example they play rugby against other schools most weeks, swim in the school pool every week, have outdoor conservation lessons, a year of instrumental lessons in groups of 6 (1 term brass, 1 term strings, 1 term recorders) and do a lot of public speaking too.
For all that, it is not £4k a term better. But I knew it wouldn't be. However that is what it costs round here and we decided we could afford it with some sacrifices but are not driven into penury by it.....still have savings and can afford a couple of cheapest holidays a year......so we see it as our big luxury and enjoy seeing our boy thriving on the opportunities.
I'm pretty sure he would still have got in at 11 to the secondary. He is bright and although most places go to kids from preps many come for state too. So it is a costly exercise for us....and that is with one child!
I wasn't happy with the state primary and the thought of another 4 years of feeling disgruntled drove me to pay. If money had been a lot tighter, I would have just put up with it I guess, and I'm sure DS would have done well there, even if I had a rockier emotional journey through.
One Q to consider is if you can find a better state alternative. If you are prepared to wait a bit and go for an in year admissions place, you may well get one. Kids do move all the time and you might be a term into year 3, or so, but if you get your name down at 2 or 3 good ones (and are able to drive to them) then one will come up at some point.
If you are looking at 3 or 4 kids, the picture is quite different.