In my experience, many indie mainstream schools are not geared up for very young SEN children. They might be able to cope with the child in reception/y1/y2 but the cracks start to show in year 3. From year 3 onwards many indie schools expect a lot from very young children. By year 3, many indie schools introduce subject teachers and start to stream Maths and English. Also very young children no longer use their class room as a base for all lessons, but are regularly moving around the school from room to room for each different lesson.
For my DS, he coped in reception/y1/y2 but year 3 was a disaster. Not just in terms of learning but also things like moving around the school. He simply could not handle moving from class to class and all that entailed - eg making sure he had the correct books/pens etc for the correct lesson. Also, my DS (by no means a "runner") got lost twice in his school in year 3 - a school he had been at by then for 4 years. One time the staff knew he had gone missing, and after calling me to ask if he was with me(!!) found him. The other time he went missing, noone knew he had gone until I worked out that he'd gone missing for an entire afternoon. I still to this day don't know where he was on both these occasions. His maths, always a very strong point for him and something that motivated him, nose-dived to the bottom of the bottom because he was "streamed" into the very bottom set (because of his dyslexia) and left with a maths teacher who simply did not know how to teach maths to a severely dyslexic child. When I home ed'ed him, it took a year for specialist dyslexia teacher to bring his maths (and motivation) back up to where it was before that school's teacher tried to teach my DS.
Tbh, if I had my time again, I would use my money to finance fighting proper support in a proper ss and not subject my DS through the trauma of a prestigious indie school that didn't know the first thing about how to support my DS. But more importantly, they didn't know that they didn't know, so in their process of trying to "fix" him, did immeasurable damage to him with their methods.
OP - my DS's experience of indie school is extreme. I didn't know he had SEN when I put him in that school. If I had known, I would have asked other parents with SEN children and ask of their experience of the school before placing him there. I would never ever rely on the Ofsted report - this school was "outstanding" for its SEN. Since I've removed him from the school, I discovered that my experience of the school wasn't unique.