she might have to be either returned to Poland (Brexit has made everything very unclear to me
You make her sound like a parcel! However, I think you're fussing quite needlessly about Brexit. The British authorities will not deport Polish citizens or any other EU citizens. That simply won't happen.
Take Brexit out of the picture and just focus on your partner's health. You're a bit vague about it. You mention her mental health. Do you mean she's depressed and desperate because of her physical state or that her mental health has caused her physical symptoms because the two are rather different. I'ts my impression that she has one of those conditions like fybromyalgia which conventional medicine can do little to alleviate?
just want to speak to somebody (Polish embassy etc.?)
The Polish Embassy/Consulate will have zero interest and give you no help whatsoever.
Asf for her parents, even if they were in their full health themselves, there is virtually no support for unemployed people with undiagnosed and possibly mental health issues in rural Poland and little enough in the big cities. Poland is not the answer to your problems.
Your best option is to continue seeking help in the UK. The UK is one of the best places to be in terms of access to quality medical care at little or no cost not to mention support groups etc. Is there no support group for people with a similar condition to your partner's?