I confess that part of the reason I started this blog piece was to record these statistics somewhere so that I could easily find them for prosperity since they are disturbing. Maybe a lot has to do with how we define conditions that are considered to be mental illness. My knowledge is not sufficient to debate this suffice to say that if every addiction is classified as 'mental illness', I don't know how many people that will leave as 'mentally fit'. To me three things are critical to a sound mind.
First, a person is bound to feel stressed if they don't have choices. So if you seek security make sure that you have alternatives. It's OK if you love your job in a big company but if you want to be loved back - get a dog. Businesses don't love people, certainly they don't owe them a living. In the ebbs and flows of commerce everyone is dispensable. So even if you think you have the best and most 'secure' job in the world, make sure you 'what if...?' what the world looks like if this job is not there tomorrow.
Second, manage your own expectations. Never let them get too high or too low. This is particularly true if you are expecting things from other people. If the above numbers are correct you have a 1 in 4 chance of coming across some poor individual who is suffering from depression in the UK workplace. This may well affect what promises that they make to you that, in turn, may impact the delivery of your projects.
Three, make sure you surround yourself with a 'home support system' which is right for YOU. Staying in an unhappy marriage, associating with people who constantly drain the life out of you is likely to mean you are perennially tired and make you depressed! At least in your personal life don't get into a long term situation where you feel you are going home to face a battle every day.
I can't say what the mental health 'professionals' think of these points but I can attest that they seem to work for most of the successful and happy people that I meet.