If you asked the traffic warden and they said you can park there then you should write this in your appeal.
Most council always reject your appeal first time round. Also contact the CEO of the council and tell them what happened also mention the Equality Act and tell them you will sue for disability discrimination (failure to make reasonable adjustments) if they dont cancel the PCN. As you are disabled and you needed the toilet or you would soil yourself this would count as extenuating circumstances.
People who break down and get a PCN get it cancelled. So being disabled and needing the toilet should get it cancelled. If they dont then tell them you will go to the papers.
Your best bet would be to contact the CEO(executive management team).