From evidence given in the Alfie Meadows trial:
Mansfield [for the defence] questioned [Commander] Johnson’s tactics once again, asking him if he understood the difference between ‘high-profile’ and ‘low-profile’ policing to which Johnson replied ‘I use proportionate policing’. Mansfield referred to Johnson’s handling of the G20 protest and the way ‘thing went wrong there’. Johnson said he didn’t accept things had gone wrong, to which Mansfield responded ‘somebody died there’. ‘They did, yes,’ replied Johnson.
You have to bear in mind this is probably not a gaffe (even though it should look bad to any right-thinking person). This is Total Policing. Things did not 'go wrong' at the G20, at least not for the police, who demonstrated their power and, through that, the power of the British capitalist class. Somebody died, but people do that all the time. The point was people were made that little bit more afraid of expressing their politics.
Things did not go wrong for the police on Westminster Bridge either, where several thousand young people were corralled in sub-zero temperatures and Hillsborough-like conditions by commander Johnson's men. It was revenge for the humiliation inflicted on the police by the students invasion of Millbank Tower.