The man (most likely Nigerian by his accent/voice) in the video was accosted at Kings Cross Station on Thursday 10th March 2016 by British Transport Police Officers. His offence is not known but the police officers repeatedly asked him to get down on the floor. The man was willing to be arrested and taken away peacefully but the policeman kept on insisting he should get down on the floor when he was not a threat to the police officers or other individuals around the scene of the incident.
I think the policeman's insistence on him getting down on the floor was a bid to humiliate him. Eventually, the man was held by about 5 police officers while the policeman who had insisted that he get down on the floor handcuffed him before he was led away.
I think the police were a bit heavy handed. He had surrendered to them and was willing to be led away. What was the rationale insisting repeatedly that he get down on the floor when he had no weapon on him and was not a threat? It is not as if he was wielding a weapon like the man in this video.