My perceptions on stanbridge earls are just based on the lack of parental involvement compared to the others I went to or has taster days at. But it is a lovely school even If I personally thought it was more appearance based and focused more on what I perceive to be none complex learning differculties.
From both your lists it does sound like any in the cambien group are not what you need so grateley house would be unsuitable shame really because I have two dc's there and both love it.
Rowdeford is also out for the same reasons as grateley
But iced you may wish to look into this one
www.springfields.wilts.sch.uk/index.php?option=com_content&view=article&id=47&Itemid=53
Its a very well respected school don't be put off by it also being a sports academy it is also academic
They have a max of 8 to a class. They always used to let day pupils as well as boarders but its worth asking as there policy may have changed now.
Both of you may want to check out
st-edwards.hants.sch.uk/
As it does fit all criteria mentioned, class sizes start at 2/4 to a max of 6 but only when child can cope with that many a class of 6 has 2 staff members. They have about 50 kids in the school and all ASD kids have there own tailored social plan, it's also very academic and fun. I currently have one HF ASD ADHD SPD delightful dc there he loves it, there is not one thing I can fault about it, ( despite being totally up my own arse and liable to pick fault).
The differculties you will have in this area with as spersific schools is we don't have many, the ones we do have tend to cater for the same children rowdeford school does ( www.rowdeford.wilts.sch.uk/home.php ) as all the other kids the LA try's to force into mainstream or downlands despite closing nearly all the ASD units in the mainstream schools.
But this one. www.yell.com/b/The+Mark+Way+Special+School-Special+Schools+and+Colleges-andover-SP101HR-5534/
Is a very happy medium between independent and state schools, the school building when I looked at it was fairly shabby but the teaching and pastoral care was incredibly good,they also had a very good quite safe room and classy sizes were 8-10.
This also fits both your requirements
I know children at the Andover one who are very happy there.
Larkrise in Trowbridge is exerlent but Its more aimed towards sever learning disabilities