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The man was restrained, he had both of his arms grabbed by another man and crossed. The reason he got headbutted was because he was shouting about his own inelegance
"I'm not clever I'm not clever"
Maybe he realised his stupidity? He didn't seem a threat in the video, and he was in that position for over 5 minutes according to sources.
The headbutt was not essential, or defence, or reasonable force. Therefore it is assault plain and simple.
One rule for us.. another for the biggest legal gang in the UK.
Just because someone is part of the police force does not mean they are morally correct.