Deconstructing a few paragraphs from this source, here's what happened according to Assistant Commissioner Simon Byrne:

 14.20 - first 999 call about a man being attacked, but with no mention of a gun
 14.24 - first 999 call that mentions a gun, armed response unit assigned
 14.29 - first unarmed officers arrive at the scene, establishing a cordon and waiting behind it for the armed response
 14.34 - 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.