Guy - it was good of you to expand on your thinking but I really think you have missed the point. no one here is saying that FT wouldn't be best and of course a 4 year old can go to school for 6 hours per day from the outset. A choice of full days or half days would be even better and maybe your particular school will chose to do this in the future. The issue here is however that your school has chosen to do have a protracted settle in period. It will have its own reasons for this but the issue is how you are handling it all...
You may have support from many (including me) that schools should offer FT schooling from day 1 but you don't have the support in terms of how you are handling this issue. Insisting that you will be approaching the school to exercise your legal right to FT childcare from day 1 is both disrespectful to the school and your child.
I think this thread has run its course but I hope you take something from it and think carefully about the consequences of your actions.