Several points need making here. Firstly and most importantly, you don't fly if fatigued. Therefore, you have to follow your company's protocol for informing that you are not available for work. If undue pressure is then placed on you to work when you are not fit (or an invitation for tea and biscuits), then file a Chirp report and also make sure your union is on your case. Should you be at work and find yourself fatigued then make a Pan call, write an ASR and other such paperwork. You may even have to divert. That will get their attention. However, you should have been clear in your own mind that you were convinced before departure that you could safely complete the flight.
Secondly, your company owes you a duty of care regarding the hours you work vs your trip home and are duty bound to make sure that you are not so knackered you can't safely drive home. Again, another union matter.
Lastly, you are a Union member, aren't you?
PM