As someone who also works with people with intellectual and developmental disabilities, I can’t disagree enough. In my experience, people-first language is strongly preferred. You delineate between what is considered the preference in Britain and in the United States. From my perspective, it’s far more than half and half here in America. Most organizations here go person-first.
And there are solid reasons why. Numerous studies in disciplines across The Humanities confirm the importance humans put on primary information in a cognitive sense. The first word mentioned gets the most attention. So, if someone says “disabled person,” they are featuring the disability first in the mind of the recipient of the message. If they say “person with a disability,” personhood is the first thing the recipient thinks of.
You can disagree, but, as I said, smarter people than me have done the studies.