DH & I are farmers and our feelings on the incident are quite mixed.
Whilst the footage looks disgusting when you actually consider what the alternatives were I think I can understand why that decision was made.
Animals aren't born bucket or halter trained & it doesn't matter how experienced the police/vet/farmer is if the animal is not used to being handled then it's going to be virtually impossible to safely restrain it.
The only alternative would have been a marksman (or armed squad) in an urban residential street which would have been more dangerous, more distressing for residents & wider public & ultimately killed the animal.
We had our fallen stock man come this morning who is also trained marksman & he was saying there was no way he would have been able to shoot in that situation.
The police officer probably feels terrible about the whole thing & extremely unlikely to have done it in a moment of anger of impulse it would have been a carefully planned maneuver. It's something police have done before so assume it's a known driving skill.
Having said that as farmers its drummed into you from the start that you don't cause suffering to an animal. Where you need to take a life you do so as quickly as pain free as you can. I have never seen a farmer do something like that, however we are in a rural area & not south west London (staines/feltham is technically Surrey but if you are unfamiliar it is very much suburban London)
I'm sure there will be learning points from the police investigation & its absolutely the right thing that Surrey police & IOPC are investigating.