Should I disclose this to the universitys i've applied to?

UCAS have given you poor advice I'm afraid. You need to declare all places of education from age 11 - including universities (although if you haven't received any qualifications or passed any credits/modules then you can only declare your attendance and not the award).

When you start at another university your previous time at university will be disclosed through other means (so making it clear up front is better than getting caught out withholding information).

Given the circumstances disclosing your attendance will only have a positive affect on your application.

I left university 3/4 of the way through my first year due to health reasons and therefore I didn't receive any qualifications from there. I've reapplied to go back this year, but when I was filling the UCAS form out I called UCAS to see where I should put that i've started at uni before. The person on the phone said that I didn't need to put it on as I didn't receive any qualifications.

However, i've been invited to some interviews and i'm worried that it'll look bad that I haven't told any of the unis that i've started before and they'll take any offers away. Should I email the admission departments and just let them know that i've started at university before just so they know?

You don't need to put it on then.

It wouldn't harm to email the admission departments, but I doubt it'll do much benefit either.
If you do email them, say why e.g. health issues/complications, in a short, concise manner.
I think that would be favourable.

Mind, I am no admission tutor, nor do I have much experience in this department (my disclaimer.)

Edit: PQ above looks like (s)he knows what (s)he's talking about, so ignore this.

She really does. Nobody knows more than PQ.

Apart from Hannibal Lecter.

(It was the first thing that came to my head, and I still decide to publish it.) <

Your entitlement to student finance won't be affected in the sense that your 'gift' year has gone but you will still be funded for the whole of this degree, but if you drop out again, then you will have to fund the first year of any subsequent study yourself, just so you are aware of that. Student finance will certainly know that you have attended university before.

Because the OP left for health reasons, (rather than dropping out because they didn't like the course) they would be, with the appropriate evidence, entitled to compelling personal reasons. This means that the "gift year" is ignored.