Right, so there was some very important info missing:
The patient did not have food and fluid withdrawn, they voluntarily stopped eating and drinking.
That's legal, and as long as you have the mental capacity to make that decision then nobody can force you to eat or drink. It's a slow method of suicide but suicide is legal.
And as long as you have mental capacity it's also legal for HCP to administer palliative care in that situation, as long as that care does not amount to assisting your slow suicide, i.e. they do nothing to actively hasten your consciously chosen death by self-starvation.
As others have said, the mental capacity act and the mental health act are both quite complex and the way they interact can be confusing, so maybe it's theoretically possible that a person with a serious mental health condition but nothing much physically wrong with them could nevertheless be assessed as having the mental capacity to make this particular decision - to starve themself to death.
But I think any such case would have ended up in the courts and we would all have heard about it.
There's a page here about VSED and your rights:
https://compassionindying.org.uk/resource/voluntarily-stopping-eating-and-drinking-vsed/
'100% of health and care professionals with experience of responding to questions about VSED would find it helpful to have guidance on the legal and clinical aspects of it.'
Read the case studies on that page. Even people in their 80's or 90's who are in very poor and declining physical health, with no mental health diagnosis, sometimes end up being sectioned when they decide to voluntarily stop eating and drinking.
So I do not believe that any doctor, faced with a patient who is seriously mentally unwell, but physically well, would be happy to proceed with such a treatment plan without the backing of a court ruling on their mental capacity. And such a ruling would be important case law so we would have heard about it, just as we have heard of Diane Pretty and Tony Bland.
The usual thing to do for a seriously mentally unwell person who wants to starve themselves to death is to admit them to hospital under section for their own safety and if necessary to treat them against their will.
Nobody likes the MHA when they're being sectioned but posters here have shared their stories of how, with hindsight, it has kept them safe and helped them recover.
It's also true that a very small number of people never, ever recover and are never well enough to be discharged and spend their whole life trying to kill themself. But as the law stands we don't say, 'OK, you've done X amount of years on section, off you go home now to end your own life if that's still what you want.'
We can debate whether that should be the law but that is not the current law in the UK.
So I think there is still some missing info here, or else you have discovered Shipman V.2.