You can be dismissed without reason at any time. (If you have been employed for 2 years or more you may be able to claim unfair dismissal if you are dismissed.)
However, if you are being discriminated against you can make a claim against your employer at any time during your employment provide the discrimination is in relation to one of the protected "characteristics". These include but are not restricted to age, sex, sexuality and race. The Citizen Advice Bureau give an outline of discrimination here:
http://www.adviceguide.org.uk/england/discrimination_e.htm
If you are suffering illness as a result of the stress that you are under you may be able to make a claim against your employer. For a summary of the law see:
http://www.lexisnexis.com/uk/lexispsl/personalinjury/document/393870/55KG-FP91-F18H-K2JH-00000-00/Psychiatric%20and%20occupational%20stress%20claims%E2%80%94overview
You would need to consult a solicitor if you were minded to take such an action.
On a practical note I would advise that you inform your employer of what is happening either through your union (if you have one) or to your manager's superior. I understand that this can be a difficult thing to do but if you do nothing then nothing will change.