Cheapandchic. Re your OP most councils need proof of your address, so council tax bill, gas etc they need to be less than 3 months old, so re timing you woul need to move in the summer to be there for application date, (can't remember when but think it's oct/nov).
You find out about places around Easter and then start school in September, it would be very foolish to move after offer but before start of school, so you would probably need to stay to nov/dec so knowing most rental are 6months or 1 year you would need at least 18 months.
My local highly oversubscribed secondary school sends a letter in July which has to be signed by the head teacher of your primary school to prove that you live in the address you applied from.
I will not get into the wrongs or rights of this but feel that many of the other posters have pointed out the legal status of this, which you do not seem to agree with, but it is the reality.
The only other thing to consider is if you are applying to a very oversubscribed school with a really small catchment area, other parents will judge you, you may not like it, or care, but you will have to live with it for a long time.