do you have to mention mental illness on UCAS?

is it necessary? what if you don't want to - is this okay

You don't have to, but it's generally a very good idea to declare it because it allows universities to make provisions to help you. If you don't need their help then that's fine but if you would benefit from help, they're only able to help if they know you need help in the first place! They definitely will not discriminate against you because of mental health issues.

okay, but say if they assumed that because of mental health problems i couldn't cope with the course - would it be safer not to declare it on ucas and then, once i have all my offers, tell the universities i'm firming/insuring that i will need help when i start the course? i just can't think of anything worse than being rejected because of a problem i can't control (i'm sure this won't happen but it still scares me and i don't want to take any chances!)

I don't know what your situation is, but I made Oxford, Imperial and UCL aware of my issues from the very beginning and that didn't cause me any problems whatsoever. At least in my experience, the tutors seem to be very interested in doing everything possible to make the course accessible for the people they give offers to (particularly at Oxford because I've had the most contact with them but the support at Imperial and UCL also looks really excellent, the only place I've had a less-good experience with was Southampton). It's your choice of course, if you don't need any support during your interview then I suppose you could tell them later if you get an offer but I really do not think that there's any risk of you disadvantaging yourself by declaring it beforehand.

okay, thank you
i'm not applying to southampton but i'm sorry to hear that their support wasn't great. but i'm glad oxford's is. i suppose it varies with college though

Possibly, a lot of it is centralised though and I've been very happy with the support I've been given from the central disabilities advisory service and at a departmental level too, not just at a college level. I honestly do not think that you are going to harm your application by declaring a disability (it really is just for your own benefit to get the help that you need).