So if you slap cuffs on someone and took them to the station without presenting the announcement they were arrested, then what? Surely, you've denied that person rights of knowledge of caution?
"What MUST happen: Officers only need to inform you that you are under arrest and state the grounds (the specific offence you are suspected of committing) as soon as reasonably practicable."
To me, that leaves ground for the 'self-defence' that the jury found where there's no cuffs on and no statement of arrest, just a reaction to what was happening to the family member.
Nothing else makes sense.