"9.79 If a child’s parent or a young person makes a request for a particular nursery, school or post-16 institution in these groups the local authority must comply with that preference and name the school or college in the EHC plan unless:
• it would be unsuitable for the age, ability, aptitude or SEN of the child or young person, or
• the attendance of the child or young person there would be incompatible with the efficient education of others, or the efficient use of resources
Efficient education means providing for each child or young person a suitable, appropriate education in terms of their age, ability, aptitude and any special educational needs they may have. Where a local authority is considering the appropriateness of an individual institution, ‘others’ is intended to mean the children and young people with whom the child or young person with an EHC plan will directly come into contact on a regular day-to-day basis. " SEN Code of Practice page 172.
The parents would carry most weight, unless the LA can prove that there is an incompatibility with the efficient education of others, or that it would cost far in excess of the cost of a special school place to place the child in mainstream.
That means that generally, the parents would get what they want. But honestly, most parents don't realise the benefits of special school places, and by the time they do, the schools are full and they have a massive fight to get a place that they could otherwise have got in the first place.