Informal PIP being enforced like a formal PIP, what are my options?

In England. I have been put on an informal PIP which is being enforced as a formal PIP with a feedback meeting and a capabilities meeting, with the risk of termination. What are my options to combat this? I have been given KPIs but no actual plan for improvement. I have been given a verbal summary of feedback but not provided with written evidence to support this either. What action would be the best course for me to take?