In England there's the charity "Time to Change" (formed by the two charities "Mind" and "Rethink Mental Illness") who tackle mental health stigma.

They aim to get people talking about mental illness and mental health, and this includes employees and employers.

http://www.time-to-change.org.uk/

Here's their stuff about work: https://www.time-to-change.org.uk/time-to-change-your-workpl...

Most of that is aimed at traditional employment, rather that start-ups.