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First of all, you appear to be using your name as a forum name, we cannot recommend this as it may open you to unwanted messaging and even spam. See
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum You refer to a PCA, this is a term used by Incapacity Benefit, the relevant one for ESA/UC is a Work Capability Assessment (WCA).
1. The simple answer is that they are different benefits looking at different things. UC looks at the limitations that would prevent a claimant from working, whilst PIP is aimed at providing help for the additional costs that a person with disabilities or long term illness might have.
2. It is not the condition that is being assessed but the limitations that result, so no there are no automatic exemptions from assessment other than for certain types of cancer treatment.
3. The Assessment Providers may have decided that an assessor with broader experience is required.
Gordon