If you have been issued with a disciplinary sanction or been fired, you have the right of appeal.
Whether you appeal or not will depend on what the disciplinary sanction is and why it has been issued. You should only really appeal if you feel that the sanction is unduly harsh or if you consider your employer has breached either its internal policies and procedures or the laws of natural justice, as set out in the ACAS Code of Practice.
If, for instance, you have been issued with a written warning for accessing porn at work (yes, it has been known to happen), you might want to appeal if you believe the company did not follow its own internal disciplinary procedure or if you feel that the sanction was unduly harsh because, for instance, your employer did not take into account your previous unblemished record.
If you decide to appeal, remember you will need to cite your reasons for appealing. There is not much point appealing just because you don’t agree with the disciplinary sanction. You must have a reason.
And yes, appealing because you intend to sue your employer for firing you is one such reason.