When I said "voluntarily" I did not mean they could go along on their own. My mum lives in her own home and has lost a lot of mental capacity and has frequent quasi-psychotic episodes. She refuses all help and all treatment and has no professional carers, she administers all her meds herself which inevitably goes wrong. This means the only outside help she gets is when members of the family or neighbours rush round most days in an emergency but she often insists they go away because she doesn't want any help.I think you are underestimating how I'll someone has to be to be sectioned. There is absolutely no way my mum would have been able to attend anywhere voluntarily. She was psychotic and thought that everyone she encountered was trying to murder her. I cannot explain to you how at risk she was but also how at risk I was as her carer. She was violent and psychotic and my life was in danger. Sectioning is taken very very seriously as you are depriving someone of their liberty.
With enough careful persuasion from various trusted friends and professionals it may be possible to get my mum to accept the need for an assessment. That is what I mean by have an assessment "voluntarily".
In my mum's case her circumstances get bad almost every day, so if she refuses all the cajoling and encouragement then the CPN and the Old Age Psychiatrist may have to section her to assess her. After she has been sectioned, it seems that S117 would pay for her nursing home care but not if she accepted all the advice she had been given to be assessed. That is a huge difference.
For people who don't have dementia, I believe there are various other rights a person has if they are sectioned compared to another another person who chooses to be admitted to a mental hospital. On the other hand I believe a sectioned person can't refuse treatments in the way a voluntary person can.
I'm open to more information about this process as I am still learning about it.