Informal exclusions are illegal, and their job is to educate all pupils placed with them, not just the ones who "fit" their environment and don't need help and support to do so. Breaking the law is hardly setting the best disciplinary example to their pupils, is it?
Actually, by your own admission, no it isn’t. If they cannot meet that pupil’s needs in their environment without it interfering with the education of the rest, they can say so. “Reasonable adjustments” does not mean “any adjustments needed, including changing your whole set-up.”
Not the same thing at all. I haven't "admitted" anything, I've simply referred to the undoubted fact that, if mainstream schools can show that they fit the legal criteria, they are very occasionally entitled to say no to a child with an EHCP - although that could not conceivably account for an EHCP rate that is 10% of that of other schools in the area. If you doubt the legal requirements involved, have a look at this Upper Tribunal decision for an example of what is expected.
However, issues about admitting children with SEN self-evidently have nothing whatsoever to do with so-called "informal exclusions" of children already in the school, and which are unquestionably unlawful. Schools are entitled to say they can't meet children's needs, but that does not in itself entitle them to exclude them illegally.
And well done, you've clocked what the term "reasonable" means in the phrase "reasonable adjustments". I've never once suggested that schools should make any adjustments over and above what the law expects. The law does however require schools to go to some effort to give effect to parental preference for mainstream education, and if that is going to be incur costs, e.g. in extra 1:1 support, it requires local authorities to provide those costs. If it means the school applying its ultra-rigid discipline and uniform rules in a non-discriminatory manner, the law absolutely expects that.