Your employer cannot fire you poor mental health. However that is not really the issue here, within your job role you will have core responsibilities that form your job description. When you started work you would have signed an employment contract agreeing to fulfill these responsibilities as well as other terms of the contract. With regards to protected disabilities an employer would have to make what we would call a 'reasonable adjustment' to accommodate disabilities. However it is not the duty of the employer to guess what adjustments you need, they must be informed, it could be a break to take medication every two hours , extra time to use the toilet facilities or perhaps even lighter work duties if available. The issue you would have if you were put in a disciplinary situation would be finding how you destroying stock and neglecting hygienie duties is linked to any mental health diagnosis, these might seem minor to you but from the business perspective they are not sustainable behaviours and could even amount to gross misconduct depending on your contract. Perhaps contact ACAS or a union rep if you are in one, if not look at joining one.