Thanks for the replies. The course is social work. I was sectioned six years ago for attempted suicide, so does not have relevance now and I was not diagnosed with anything.
TBH, I doubt it's the fact that you were once detained under the MHA or previously attempted suicide that is the issue so much as the fact that you did not disclose it. When I applied to do the MA SW, I disclosed past mental health problems: all that happened was I had an extra appointment with Occupational Health who cleared me as suitable for the course.
You're very clear that as you understand it, you did not lie, because you have not been diagnosed with a mental health condition. I imagine the uni are coming at it from the perspective that it was lying by omission. You don't have a diagnosis, but a significant past suicide attempt and detention under the MHA is very relevant information that you should have disclosed- and after disclosing you could have demonstrated that the issues you were dealing with at the time are no longer impacting on you, and that they have no bearing on your ability to safely practice.
Although they don't register students, the HCPC do provide guidance- have you looked at that? The university may refer to the guidance on conduct and ethics, particularly "You should provide, as soon as possible, any important information about your conduct, competence or health to your education provider and practice placement provider." The uni are likely to regard the fact that you were detained under section following an attempt to kill yourself as important information about your health.
If you have student membership of BASW or Unison or another union, make use of it and get their support in dealing with this.