If you are going to use state primary, have a frank discussion with the head-teacher about competitive entry to private schools at 11+.
I did this and got a wide variety of responses - from a look that suggested I had asked whether they practiced satanism and it being made clear that they didn't really approve of private education and would not be helping in any way, through the 'never really thought about it's to the 'oh yes, we send lots of our children to selective schools every year' and a list of the latest placements and information of the 11+ prep classes arranged after school!
Obviously we are now sweating it out on the waiting list for the latter!
If your school is very supportive and high achieving then I think you could either stay there throughout and not dent your chances at the big London day schools or similar.
If you want one of the big schools - Eton, Win Coll etc - then I would suggest state until 8, then take a look and either try for a prep place or if things going well then maybe hold off till 11. I wouldn't try and do a state secondary entry to one of those schools unless you know exactly what you are doing and have some serious help with tutoring.
FWIW, I went to a hot housing prep and then a grammar. The difference between the two in terms of all round education was HUGE. At the time the grammar took the top 25% of the local comps (was 13+ entry) and the prep kids basically marked time for the first year - I believe it's now a tiny number from the local comps and mainly prep kids going there so things have probably improved.
My prep had specialist teachers in every subject from Yr3, huge grounds, sport 4 afternoons a week and activities the other. I wasn't happy there for various reasons, but I can say it was the best education I had at any school - we were not just taught to the test, but way beyond and sideways.
That said, there are a lot of preps which are just expensive versions of a mediocre primary.