Can't sleep!
Came across this which I found interesting.
The DLA actually did include MH as an impairment. You would have needed to be significantly effected. Psychosis was a common reason for receiving benefit.
My assumption was that learning disability, deaf, blind, wheelchair bound would have been the highest, but it looks like it's arthritis.
https://www.theguardian.com/news/datablog/2012/jan/18/disability-living-allowance-data