You would think you would be able to tell someone you're thinking of self-harm or demonstrate it without hurting yourself and they'd understand. I've put this into practice IRL by not actual SH and haven't had great results, but it could depend on who you tell or how you do it. I think part of that is people just don't empathise with suicidal ideation or self-harm very well.
So to get help instead, you could try telling someone you have suicidal ideation and/or go to A&E if thats an inexpensive option and doesn't endanger your liberty(e.g. section or worse). Safeish option that should only expose emotions is talking to the right people might help, either a friend who understands/has been through similar or talking to someone on here. There's the general public chat on here you could try that.
Another way might be to distract yourself with a hobby, if you can do that, but it's understandable if you feel too bad to do that. Also 'cup of tea' was mentioned in a round about way that might actually help, tea or a drink can help to feel better sometimes.