If it's a person's hospital records, you would approach the communications, or the patient access, team of the hospital(s) who should be able to point you in the right direction. You'll have to fill in a form and provide patient consent, or in your case proof of PoA. I should imagine the process to be similar with GP surgeries.
Every patient has the right to access their medical records, you provide proof of identity.
I am feeling much better about this issue as I called in to Mum's surgery today to ask several questions on of which was to explain about the form I needed to fill in and my lack of information.
The lovely receptionist said that rather than having access to the notes which would mean trawling through them while someone stayed in the room with me, if I take the form in the will fill in all the relevant bits.