@kitation
Also having someone with her in appointments can be a way to minimise dismissal and back her up.

Fibromyalgia is a diagnosis of exclusion, so make sure they have actually made an effort to exclude other causes. If she suspects a particular problem based on her own research, she has a right to bring that to a doctor and have them listen.

The NICE guidelines doctors are supposed to follow are accessible online (https://www.nice.org.uk/guidance/published?sp=on). If the doctor is not following them, ask why.