This report has a chart going back to 2007. It looks clear to me that in the early part of the 00s, we were getting to a good place with recognising and supporting SEND and then all the cuts/austerity came in and diagnosis numbers went massively down, as well as provision and support. But the numbers of pupils needing such support, logically they didn't go away, did they? They were still there but not being given the required support. Now it looks like numbers are rising massively but they aren't really if you look at this chart - they are catching up to where they were previously. So I think if there was to be an inquiry the first question needs to be whether numbers are actually rising or whether we are simply recognising better. (I understand that the numbers actually say "it's both").
https://assets.publishing.service.gov.uk/media/64930eef103ca6001303a3a6/Special_educational_needs_and_disability_an_analysis_and_summary_of_data_sources.pdf
Worth noting that SEND does not only include diagnosed disabilities and learning disorders - it also includes SEMH (social, emotional and mental health) which used to be called things like "behavioural needs" and "disadvantaged background" - basically, it's covering any pupils who are likely to need extra support in school whether that's because they have a learning support need like dyslexia or they struggle with school due to having missed a lot of school, or because their home life is stressful or because they have never been given positive role models, as well as issues like ASD and ADHD and more profound disabilities ie pupils who would never have been in mainstream schools.
The Warnock report is a good one to look at too. This was from 1978 and estimated that 20-25% of all pupils would need some form of SEN support. So these numbers aren't new. Some history/development since Warnock here:
https://www.frontiersin.org/articles/10.3389/feduc.2019.00164/full
What actually seems to be the case is that up to the early 1990s or so, all of these issues existed but they weren't very visible to the average person. There were absolutely institutions, there were a lot more special schools, children may have been kept at home (similar to today) if it was felt they "couldn't cope" with school, but also it seems like pupils in older age ranges were largely left to it - it's hard for me to know because I was too young, but there are anecdotes all over - pupils truanting and a letter was written to the parents, but the pupil intercepted it easily so their parents never knew. Children were left to their own devices a lot more and it was seen as their own responsibility. Pupils not passing exams but it was not seen as a societal issue but a personal failure of the pupil. There are adults today who left school, legally, of age, with no qualifications but in those days nobody was totting it up for a spreadsheet and reporting it in the newspaper. It was taken for granted that some pupils wouldn't pass exams.
It seems to me like people saying "It's too expensive, we can't support such a high number, we need to know why the numbers are increasing so much!" are saying that it was better in the days when we just ignored that middle section of SEN pupils and we should go back to not trying to meet their needs at all.