Schmoobarb indeed that is my understanding too, that stress and mental illness might look similar in some employees but they are not the same. If you deal with/manage stress, you stand a better chance of it developing into a mental illness.
Hence the variation in some GPs being much more amenable to signing people off with 'stress' than others. The NHS website says it falls within generalised anxiety conditions and medication won't be offered automatically.
So while I'm happy to go and read up on it, I don't accept that I'm way off beam, sorry Snap and Cards. Stress is real but it can be eased considerably by e.g. removing one of the sources of stress rather than by medical treatment.
In Freewheelin's case, of course her GP was right to sign her off because her employers were being dicks and expecting her to be in three places at once.
In my colleague's case, I sat in a pod with him the week before he went off. There were no signs at all of stress, he seemed as amiable as ever. The sign off did however coincide with some of his work being criticised and so it irks me that I feel possibly he's using a situation which has caused me massive unhappiness and anxiety, to support his claim of being stressed, when I have been really, really low over the same period but just kept going.
Anyway OP I really hope you find a way of getting your employers to address the stress in your team as a whole, good luck