@daytoday I really do not see how you can say "there are no other schools" For example north of Coleridge there is Rokesley and Weston Park; South of Coleridge there is Ashmount (5 minutes walk from Coleridge, in Islington) and Stroud Green School. These are all from memory so I may have missed some out, also I am not listing the church schools.
Coleridge was enlarged because there was a big black hole for schools in that area. So there are no other schools. I wouldn't move south of coleridge (away from Crouch end).
I do think there are a lot of children who leave in reception and early year 1. Many families finally make the decision to move out of London. However, I would recommend calling up each school - they will know how many they have on the waiting list still and also, they may informally know that someone is moving out but hasn't handed in their school notice yet.
I'd give the school and ring and ask them. Although it's the LA who manages the admissions, they will have an idea of the movement in the school and possibly of the current waiting list situation ie how close to the school you'd have to move to be near the top of it.
We are moving to Crouch End this June and will submit an in year admission application for our son who will need a year 1 place starting in September 2013. We plan to move near Coleridge because we thought that with 120 students per year there is more chance for a vacancy to open up. We would also be happy finding a spot in any of the other state schools within easy walking distance. Does anyone have an idea about the turnover between reception and year 1? Is this a bad plan? All suggestions welcome.