How many people have ever been diagnosed and treated by the NHS or private mental healthcare systems at any one time in the UK

This would be a figure which would interest the media and the public if
they want to get an idea of just how crazy the British nation is.

It is likely to increase every year because a larger percentage of
people are using the mental healthcare system as time goes on. It has
probably grown in funding faster than the population has risen. More
people and a larger number of types and a larger age range are accessing
it as time goes on. At least I guess so.

Anyway, if the figure is anywhere near a half for the lifetime
prevalence of mental illness (by definition of accessing NHS or private
or charity support services (e.g. The Samaritans) mental health care at
any time in their life) then a lot of people need support from mental
healthcare services.

But what is mental health and what does it mean that (perhaps) half the
population's mental health is bad enough in their lifetimes that they
have to seek additional support from a formal mental healthcare system?