Based on Mumsnet contributions and the fact that private schools are providing a 'public good'(in the economic sense) here are the first few clauses of a code of practice that such schools' collective bodies might institute. (Please add your own clauses).
We being members of the HMC, GSA, GDST etc follow this code:
Our charges for registration shall only cover costs of administration and the exam marking;
Offers will be made in line with national offer day timetable;
If we take your child we won't kick them out at end of GCSE's unless for behaviour or attendance reasons;
If we offer your DC a reserve place we will tell you where on the reserve list you are;
All children can receive their marked exam papers within 8 weeks of offer day (after all their yours and you did pay for them);
When we interview your DC we won't ask where else you have applied to;
We do not interview parents;
More to follow??