Trigger warning: Girl gets arrested, seemingly for being Autistic in a built up area.

I'm still confused about what really happened or how the situation could have been handled.

1. The police were first involved in taking the drunken girl home from the city centre.  Why?  I thought the police were  no longer getting involved in mental health issues.
2. Why was the mother allowed to film the encounter on her mobile phone?  Any competent police person would have confiscated her phone and deleted the incriminating video.

Confiscated the phone? Under what authority?  We are not yet a total police state.

Ian said:

Confiscated the phone? Under what authority? 

https://www.jointhecops.co.uk/how-long-can-the-police-hold-your-phone/

In order for a police officer to seize your phone, they must have either been granted a warrant to do so or after an official arrest. 

The police in the UK cannot legally obtain your mobile device without good reason. Therefore, if you find yourself in a situation where the police are involved, you do not need to hand over your mobile phone without a warrant or before an arrest.

The UK PACE caution is  " Anything you do say may be given in evidence."  That means the defence can use the interview to support their case.

I thought strictly speaking it was an adverse inference from saying something in court that you didn’t say previously. For example if you say ‘I couldn’t of stabbed him because I was in such and such a place’ in court when you never said it in the police interview.

this is why lawyers will have their clients go to the police interview read a statement that covers all of the facts that might be useful in their defence and then refuse to answer any questions. Because that way they have actually mentioned at interview things they rely on later at court. 

I thought strictly speaking it was an adverse inference from saying something in court that you didn’t say previously. For example if you say ‘I couldn’t of stabbed him because I was in such and such a place’ in court when you never said it in the police interview.

this is why lawyers will have their clients go to the police interview read a statement that covers all of the facts that might be useful in their defence and then refuse to answer any questions. Because that way they have actually mentioned at interview things they rely on later at court.