When I read your post I had several thoughts.
First, is that is very difficult to know what your DC would be by Y3, so choosing a prep school that you think will work for him at 7+ is difficult, and not really possible. He will change so much between now and then. So, if I were you I'd choose the best school for DC for right now.
Next, just like MMM above - going from a state system to do a highly competitive 7+ is not easy. It will require a lot of extra tutoring, and would not be pleasant for you and your child. A friend just did that, unfortunately unsuccessfully, and I watched them with sadness...
Finally, from an educational standpoint, I love the Montessori approach for the early years, provided that it is done properly. It works wonders for a curious child who is eager to learn. A lot of research indicates that too much formal tuition in math/English is actually hurting long term educational outcomes. Montessori allows children to move at their own speed. Both of my DDs went through a Montessori nursery and came out reading, with good feeling for numbers (both small and large) and with solid understanding of the world. If there was a proper Montessori school in the area, I'd seriously consider it.
HOWEVER, the reality of this country, is that Montessori system is not developed here, so you'd still need to go back to mainstream middle/high school. And that means taking exams either at 7, 8, 11 or 13. So, before committing to any school - I'd check where their kids end up going for their next school.
Good luck!