He considered that five youths to be a considerable threat to his safety, so according to the news, if read correctly, he backed out and say the letter opener, which that to hand, he went out and fought them over, stabing one of the individuals five times before he ceesed attack and the others felt threated enough to bail.
So the weapon was "to hand" but the fact it was a knife will not work in his favour because of the various "knife crime" campagins that have been going on, the fact that the indivudual that took the brute of his force needed to be stabed five times also will cause misconceptions. A stabing is considered to be very bad, however in reality most stabing are non-fatal, and superfical.
So yes, it could be considered excess force in front of the wrong jury, but I feel this will not go to trial. Futher more we will not hear that it has not gone to trial because the Daily Mail does not follow up a story if it is boring.