I haven't read all the previous posts so will reply to your initial message.
As a parent, you have the right to say to the LA which school you would like your child to go to, but they don't have to agree. At the end of the day, it all comes down to budget. If the school you want is more expensive than the one they are suggesting you will have to demonstrate that the school they are suggesting cannot meet your child's needs. You may find it difficult if the schools is an ASD specific school unless it is geared up for say MLD when your son is High Functioning.
I would already start to try and make a comparison of cost between the three including the cost of transport and therapies such as OT and Speech and Language. This will give you an idea of what kind of battle you face. If it's a small difference then you have a better chance but the more the difference, usually the more the LA will fight back. It also depends on the LA you are up against, Surrey, Hampshire and Kent are amongst the worst.
At this stage, I would definitely get in touch with SOSSEN.org.uk who can provide you with a good idea of how to play it. I would already be gathering evidence and getting myself a couple of large lever arch files and putting all old reports, statements, documentation relating to your son in date order as it will all come in handy (most recent reports at the front).
Other things to do are always email LAs, avoid phoning them or answering calls from them, because, if this does end up in Tribunal, you have no proof of what was said on a phone call but you have a paper trail with emails. Also if you are called into meetings you can record them for your benefit (as long as the meeting is only about your child) but beforehand ask for an agenda, who is attending, what is the expected outcome from the meeting etc. Make notes during the meeting and after the meeting read through your notes whilst listening to what was said, then send them minutes/notes of the meeting, what was agreed by whom by email, this then becomes part of your evidence paperwork. As the discussion will just be about your child you are legally able to record the conversation without telling them if you so choose. I have told them in the past and they tend to clam up. Best of luck :-)