Help how do I UCAS?!

Hi everyone and apologies for asking this lol, I'm dumb and very new to the whole uni application thing.

I made 5 applications via UCAS, 1 rejection, 3 still pending and one gave me an offer which I'm 95% sure I want to accept. It's conditional on my Access diploma, which I already have the grades for just waiting to go through final moderation (I'm sitting well above the required grades atm so confident it won't change enough to fail the requirements on moderation), PVG and occupational health. I have one minor conviction and a mild disability, I work for the NHS currently but as a non-registrant and they had no issue with either - can I assume based on this that I'd also pass the uni's health and background check or is it a different standard?

If I don't accept this offer right away will they take it away? I'm pretty definite I wanna accept it but might wait to see what my other choices say if it's definitely not going anywhere and there's a possibility I might fail the checks.

I can't see how to accept it on my UCAS homepage - do I just write to the uni and tell them I wanna take it?

Thanks!

Hi - it's no issue, that's what we're here for
You can't accept anything until all of your unis have responded to you. The option to respond to your offers will then available on the UCAS hub.
You've got until around June to then make a final decision - so no rush.

Do I have to wait? I'm assuming the uni I got the offer from don't know whether the others have responded to me and I don't wanna look like I'm not interested in theirs - as I said it's my top choice and I'm pretty definite I wanna accept it, I'm purely keeping the others as backup in case I fail the health/background check