Deconstructing a few paragraphs from this source heres what happened according to Assistant Commissioner Simon Byrne     first  call about a man being attacked but with no mention of a gun    first  call that mentions a gun armed response unit assigned    first unarmed officers arrive at the scene establishing a cordon and waiting behind it for the armed response    first armed response officers arrive  If correct this seems normal to me Armed police are not common even in London And if as it appears the first unarmed officers on scene had already been told they might have a gun then staying behind a cordon until the armed officers arrived and them then shooting them as they advanced would all seem standard procedure