I understand your frustration but you are also playing this game of accessing services in a way that won't bring the results you want.
You have established yourself as someone who talks as if they're suicidal with an imminent plan but then goes away and is capable of keeping yourself safe until you reappear. I understand that makes sense to you but it won't make sense to them.
It is ridiculous for them to suggest that everyone awaiting an autism screening test is barred from emergency mental health care. They seem to be taking the position that time is no factor here. And also looking for excuses to kick the can down the road.
Complaining about the state of the NHS and how badly you've been treated is not the answer. Most people in psychiatric units have a terrible story to tell but at the end of the day, if you can complain so loudly, it means you're well enough to have coherent thoughts about your life in the present and you are in some way invested in what your care in the future might possibly be like. You need to stop complaining. And please don't mention padded cells. It's not helpful to talk about psychiatric units as if they're asylums. There are people who have to stay and work in these places frequently. If you're wanting state of the art help, be respectful about what they're offering. In many cases you will be offered excellent care if you finally get in front of the right person.
Until you have been denied ECT as an inpatient you have no way of knowing if you'd be offered it.
Whoever told you that antipsychotics would turn you into a zombie needs to stay in their lane. It's unlikely to be true. It sounds like you weren't speaking to someone who felt your life was in danger.
If you're truly suicidal and you have an imminent plan to end your life, tell a family member. You will likely have put your affairs in order and have evidence of preparation. Make the family member aware of this. Have them take you to the GP if they can get an emergency appointment or the GP may be able to refer to your local CMHT. It is very important that the family member makes it clear to everyone they speak to that they are personally unable to guarantee your safety for any length of time and that they fully believe you are genuine.
After that you do not agree at any time that you feel safe enough to go home for any length of time. You state that the opposite is true. You don't complain or get side tracked, just repeat your feelings and intentions.
You may have to wait a week in A and E or a fortnight on a medical ward waiting for a bed. In all of that time, you do not agree that you can keep yourself safe outside the hospital for any length of time.