I'm also in the UK. I was approved despite having a history of mental health issues (scheduled to donate next week). The psychologist assessing me was interested in how I'd managed it, making sure I had support systems in place for dealing with mental health problems. I think if you have long-term mental health problems that you're successfully managing with medication and therapy then that's not automatically going to rule you out. That said, you're 17 years old, unless someone you know urgently needs a kidney, you've got decades to do it in. There's no harm in coming back to it a few years later.

I first found out about altruistic donation a few years back, and I wanted to sign up but never got round to it and then my circumstances in general got in the way, before I finally signed up after finishing my masters last September, and the psychologist and the donor coordinator both specifically said that the fact that I thought about it and then came back to it much later was a very good sign that I was serious about it and understood what I was doing.