You are not required to inform your employer about the nature of your illness.
At least not in the US.
I haven't told anyone about my depression, and I will never do so. In teaching, it would mean professional death.
Further, even if you consider your superiors "pals," they clearl are not treating *you* as a pal if they are not doing their job and supervising you. Keep that in mind, next time that you think of protecting them. They are not protecting YOU!
Just tell the truth; nothing more; nothing less.
I'm sorry it went so badly. I don't know the laws in the UK, though, so cannot speak intellegently. Perhaps you can get some legal advice? Here in the US, a call is free.
I am wishing you well...
Shalom!