it turns out I don't need to go for a mental assessment
Is different from
They've said " please be advised, there's no further action to be taken from the adult community mental health assessment is identified as required or appropriate at this time"
What you need or want? Is different from what they deem as their responsibility to attend to, or to attend to at this time.
There are dozens -if not hundreds- of reasons they could have sent that letter and NONE of them have anything to do with what you need or want.
Like? They don’t have the funding available, this year. Or their assessor is sick with Covid, or home on bedrest with their pregnancy, or in drug&alcohol treatment, or in he hospital following a car accident, or, or, or, or; and they cannot hire a new person for any of a zillion admin reasons. Or they’re only doing CPS ordered assessments, or assessments involving duty of care complaints, or sectioning, and whomever scheduled you to begin with didn’t understand that you’d rung the wrong department, or been transferred to the wrong desk, or they put your contact in the wrong diary after you’d rung the right dept. and the right deal… so voila merde! Form letter.
A bureaucracy never has YOUR best interests in mind. Individuals within that bureaucracy may care, and care deeply, but they will always be confined to the limits of what their job allows or insists upon.
But I could refer myself for therapy (which I've already done today).
^^^Which could ALSO mean that instead of being screwed over by being turned down for an assessment? That you GET TO

Pass Go & collect $200!!!

… referring yourself directly to therapy, right now, instead of waiting weeks/months for someone else -or a whole series of someone else’s and a funding committee, and a review board, and we’ve lost your application, please apply again by …last week… if you still want to be considered, or we shall close your file permenantly… to determine IF you should be referred.
OR If you’re going private? That the private therapist can escort you through the laser-maze of the NHS, with boxes all ticked by them, instead of spending years in queues to get boxes checked.
OR this is just your first flaming hoop to jump through in your NHS Journey Of
Excellence Incompetence & Frustration.
Seriously.
What you need & want…. Versus… what any other person, place, or thing says they can do for you? Are 2 very different things.