Back to the ADA thing. In the UK, you don't have to tell your employer unless you need adjustments. Adjustments are things like flexible hours, time off for therapy, a quieter office etc. Bit of a sore point with me - see my "Anger issues" thread to find out what happened to me when I told my employer!
By the way - you only need to tell poepl that need to know. For example, you can allow HR to know about your ptsd and meet with occupational health to discuss adjustments that would be helpful. Your manager can then be told to make the adjustments; they don't have to be told why.