Here's the thing..those Officers will not get disciplined by their superiors for using excessive force nor would the CPS charge them with assault nor would a jury ever find them guilty so guess what........its safe to assume the consensus will be that they have NOT used excessive force so your opinion is not going to change this.
The basic facts are this: the male still had the knife and there is still a very real threat to life for those Officers and members of the public, The male ignored Officers requests to put the knife down. Taser had not incapacitated him. All the Officers want to do is get the knife off him and get him in handcufffs in case he has other weapons/explosives. Kicking him in the head is fully justified when there is a threat to life which there still is while he holds the knife.
It might not look pretty to you but consider this...What if the male managed to overpower those Officers and get up? When people are full of adrenaline they easily can and it happens. What if the male managed to use the knife on anyone who tried to get close to handcuff him or use a baton?. These are the kind of things that would have been running through the Officer's minds whose main priority is to protect life by getting that knife off him and getting him un handcuffs.