So what you are saying is that the British goverment should cherry pick what mental illnesses they want to support treatment for?
"Hmmm.. bipolar disorder? Sure we can treat that.... Parkinsons? Nah, lets not".
A few professionals class pedophilia as an mental illness, and the goverments have to decide if they want to treat ALL mental illnesses or NONE, in my opinion.
I don't know if Johnson is a pedophile, but if he is, prison might not be the way to "rehabilitate" him.