..........on a 15 year old girl.

https://www.btp.police.uk/news/btp/news/in-the-courts/btp-officer-convicted-of-assault--london/

 

It doesn’t state how the 15 year behaved….. did she spit or bite? 

Give the story some context. There’s obviously more to it.  I highly doubt the girl willingly complied with the instructions she was given.

 
There was obviously CCTV as some of it has appeared on the tabloids websites

 

I've seen some CCTV which only showed some of the incident without any audio. 

The problem is, there's far too many of that age, on and away from the railway, who think or know that they are untouchable by the law.  Outcomes like this only serve to make the lives of police officers even harder.  If they think they're untouchable by the law at 15 years old, they'll always think that way.  In a years time, they'll be considered responsible enough to vote!

In this particular instance, you need to see the full CCTV of the incident with audio to draw a conclusion.  There's obviously barrier staff present who didn't consider the response of the police officer to be heavy handed enough to intervene.  Barrier staff told the two individuals they couldn't come through without a ticket yet they choose to ignore the advice and pushed their way through the barrier, a clear lack of respect.  That 15 year and all her friends now know they can all just push through the barriers and nothing will happen.