How to apply for assisted suicide?

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I want to die, I am not sure how to kill myself in a way I feel would completely work, as I consider how to make suicide work for me I'm prepared to consider how to kill myself beyond a self administered method. How does a person go about assisted suicide/euthanasia if they are not terminally ill but feel suffering to an extent where they feel their situation is beyond improvement? I'm from the UK so it's not completely obvious to me how I can pursue this