My college is playing hard ball, should I just stop attending?

I don't want to sound insensitive, but underperforming in a school subject is a very silly reason to consider harming yourself. Go see a counsellor. As for dropping out, don't. Getting 3 A Levels with a low grade in one is a whole lot better than only having 2. Plus the added stress and logistics of moving to another college wouldn't do any good for those other grades. If you've already decided that dropping out is an option, you've clearly given up or don't care about that subject, just divert your efforts towards the ones you're better at and stop caring about failing in something you don't care about. Who knows, you might perform better if you stop creating grief for yourself. Sorry for being so blunt, I was in a similar situation once and would have needed a blunt message.

My college won’t let me drop a subject I am really struggling in and it’s impacting my mental health and my other grades in other subjects and I started harming myself as a result. Should I just stop attending and play hard ball with them, and drop out altogether after messing them around?

How is dropping out playing hardball? What is the logic of that?
Get a GPs note backing you or your social worker, then they may consider it.

Dropping out is not playing hardball, I was going to say stop attending, but then dropping out is a choice and it’s not mandatory to attend college so I guess it’s not hardball. The reason why they won’t let me drop it is because the AS result goes on UCAS, so if I stop doing that subject unis will think why I didn’t continue, so I guess the only option is to leave altogether if I don’t want to continue the subject.

If you drop out then arent you sacrificing your other A levels?
I doubt unis will care.
Do you have three others?
Is this the UK?

They're not going to care if you stop attending, drop out or "mess them around". Get help for your health issues then apply somewhere else and take subjects you actually care about.