so, your ds is currently in mainstream, but failing.
you want to put him into private specialist nursery (can I ask whereabouts?) so as to not waste this time.
you are worried that the EP will then recommend the private specialist nursery, not the local state specialist nursery?
don't worry!
if it is thought that your ds need s a specialist placement, the LA will move heaven and earth to get him into a state-run one, as much cheaper for them
if you leave him in mainstream, I htink the problem will be that the EP will say that it is not really good enough, but will be ok with support while a place at specialist nursery comes up.
form experience, this will take years (I was naive enough to believe this, and dd1 languished in a ms pre-school for over a year, while she slid further and further down the list fo rthe specialist pre-school)
ime, unless the local specialist place is so oversubscribed that your ds will never rise to the top of the list in time, then the LA will prefer to place him there.
if that does happen, and the LA fund the place at the further away pre-school, you will be entitled to Transport. A whole new ball game. but basically, your ds is entitled to trnsport if you live more than about 3 miles away form the school designated most suitable for him (iirc). so, if the LA agree to fund the further away school , they are also saying they have no closer placement.
this is why you need a statemnt. if the pre-school is named on the statement, then they have no leg to stand on. if not, I suspect it is harder to prove it is the closest suitable place