Just a small point - there is no "UK" MH system.
The Mental Health Act 1983 only applies in England and Wales
Scotland is covered by it's own Mental Health (Care and Treatment) (Scotland) Act 2003
There are a few pertinent differences, including significantly different language, but they both pertain to the same processes and events. This is part of the reason why you'll see some discrepancies on places like MN with regard to how people describe their own interactions with the system.
I'm in Scotland, there are certain things I'll reference that would make no sense to anyone in E&W because the terms I would use aren't present in the MHA. There will be similar processes and provisions, but the language used to describe them, and the commonly understood terms will be different, so it creates a degree of confusion.
Basically, people know what someone means if they say someone was "Sectioned", but up here it's more commonly referred to as "Detained", because that's generally how it's couched when someone is kept in hospital after an assessment. Officially, you'll be subject to a 72 hour Emergency Detention, or if there is a CTO involved, more commonly a 28 Day Short-Term Detention.