This is a link to information on the Infected Blood UK Inquiry website (the Hepatitis C and AIDS scandal of the 80s) on how UK residents can obtain their medical records for use as evidence. It also includes template letters (further down the page).
Having mental capacity means having the ability to make a decision for yourself.
A lasting power of attorney (LPA) is a legal document which enables you to appoint one or more people to make decisions on your behalf should you be unable, for example you become unwell or have an accident.
An Advance Directive is a decision you can make at any time to refuse a specific type of treatment at some point in the future.