If you do get a diagnosis you may not be able to get POA. As she may be deemed incapable of giving informed consent. It’s up to the solicitor (or whomever you use) to decide if ‘in the moment it is informed consent’. But if she won’t sign, you can’t force her too.
You can apply for guardianship via the courts of protection, if informed consent cannot be obtained. Google it as it does have very different rules.
If you don’t have either, then legally you are opening yourself up to a nasty can of worms. Plus for some things such as utilities, they will ask for a copy of the POA so you can act on the account holders behalf, at the very least they may wish to see a letter saying that you are given permission to act or even have a conversation with the account holder to have on record that they can converse with you.
You may find that your mum May listen to reason from someone with gravitas such as a solicitor. For some reason parents always think children are 5 year olds not 50 ! Etc
It’s a really hard position to be in.

Obtaining POA does make things easier as you can get on with what needs doing rather than having to persuade her that xyz needs doing and doing this way etc etc.
POAs can be drawn up with different caveats. So, it may be that someone will have it drawn up to be used immediately regardless of diagnosis or no diagnosis- as maybe the parent just doesn’t want the hassle of dealing with banks, utilities etc. Other caveats maybe that they cannot be invoked until a diagnosis is made or a certain situation arises. You can have more than one person holding a POA or have someone nominated in reserve, should illness or death prevent the first person from acting.

There is lots of information online regarding how to obtain and implement them. IMO I would use a solicitor to draw them up. That way you cannot be accused of acting improperly. But that just my opinion. You can complete the applications yourself.

Do try to get her to a solicitor, as pp says, hearing it from an Important Person is usually sufficient. Make the appointment, accompany her, nod in agreement, exclaim afterwards at how Clever Mr Important is, and indeed Mum was Clever to have him as her very own Important Person. grit your teeth

Sigh.

As far as the Health one is concerned, you can ask her whether she wants medical decisions, should she have an accident and be unconscious, be made by the doctors or does she want somebody to be able to put her point of view.

Finance - try the same argument - if she had an accident and needed something buying and brought into hospital, without PoA you couldn't do it.

My parents were asked by an elderly family friend if they would have power of attorney if she needed it in the future. They are now implementing it and it’s made them think about the implications so they got them all drawn up and me and my brother bare named. You just lodge them, they are only activated if they become incapable.

Fil persuaded mil into setting up powers of attorney by setting up a joint one with her. She was declining rapidly and it needed doing before she lost capacity. He is first named with my dh and his sister also named. It’s all fully activated as she is not capable of understanding or making decisions.

Mil was very belligerent about it all. Fil was recently taken into hospital and it was very difficult as at first dh had no authority despite mil needing 24 hour care so he’s told his dad that unless he agreed to activate them as second named then we couldn’t help again in the same way as we dud before.

I would seek legal advice asap.
We did the 'getting POA' as soon as my Dad's dementia was diagnosed and when he still had enough capacity to see the sense and his solicitor had already had a longstanding good relationship as the family solicitor.
However, by the time my Dad got the letter from POA informing him, he had forgotten and he lodged an objection on grounds that 'my daughter is up to something'. So it went to an investigattion and the court of protection - whereby they took 18 months to decide that he didn't have capacity to take out the POA. The solicitor said she had never in her entire career had this happen.

Overe 2 years later and I still don't have deputyship for his financial affairs and things are at breaking point. I'd complain but there isn't any point and every time we exchange emails, his solicitor charges it against my Dads account. Breathtaking neglect of duty by the court of protection who have taken so long and there has been safeguarding issue after safeguarding issue. Really. So get a solicitor to advise you OP.