Location
Is it important to you to be close to family, friends or medical care? Do you want to get as far away as possible? Many students do now choose to live either at home, or close to home, and some universities have higher proportions of 'home' or 'commuter' students.
This can effect what is on offer to you - for example, if it's important for you to meet new friends, but you are going to live at home, are there lots of social opportunities that are not entirely focused on halls of residence or student accommodation? Does the Students' Union run events for students who live at home/commute to get to know each other?
If you want to move away from home, do you need to be able to access good public transport networks? Is the university city or campus based? Would you like the hustle and bustle or prefer quiet? Perhaps being by the sea or in the mountains would be good for your mental health? Or would you like to be able to access the clubs and music venues of a city? Even the ease of day to day tasks like grocery shopping might be something that's important to you and your mental health.
Size
Universities vary hugely in size. Some people like the anonymity of a large campus, but for some with a mental health condition, the support offered by a small campus population can make a huge difference. Specialist providers, such as creative or teaching institutions will by their very nature have a large amount of specialist knowledge when it comes to how mental health can affect studying their particular course. It may be that they only have a small number of support staff, and may even outsource some mental health provision such as counselling. This isn't necessarily a bad thing, but again, it is worth researching.
Large universities may have lots of staff, but that doesn't mean to say the ratio of staff to students is any better. Counselling services may have waiting lists to access them at the busiest times.
Diversity
It's worth mentioning here that cities are, by their nature, far more diverse places, which many people find liberating. It might be the first time ever that you'll easily be able to meet up with people like you in person. This doesn't mean to say though that the university itself is diverse. Students' Unions can be good sources of information about this.
Universities who do have a diverse population will often tell you about this in their literature - if you want to double check you can also look at statistics on the Higher Education Statistics Authority website (look at the 'widening participation' indicators).
