[Kate*]

Not from the UK, but I think the policies are pretty much the same. The only times they can break confidentiality is if you threaten to harm yourself or someone else. Suicidal thoughts are confidential unless you can give details of how, when and where etc. or if you threaten to hurt or kill another person, and mean it, give specific details. So, they aren't going to tell your parents about your drug use or your suicidal thoughts unless they're convinced you will go through with something.