There is usually a Stanley type knife that lives in the car, but it is currently missing...
I shall hope to not need to let you know.
I would have thought you'd follow similar process to deer road collisions. If you're able to use an appropriate firearm and if you're not call the police and they should have a list of people who can come out and do the deed. I'm not sure of the legality of using a knife although it has been suggested if you do nothing you're also breaking the law.
Edit to add this might be useful, although I doubt you'd be carrying a 5" fixed blade as you're unlikely to have good reason ("I might stumble across a wounded sheep officer" probably isn't good enough.)
if one has tree dismantling gear a 5" fixed blade is a very legitimate carry as you may need to cut oneself free of a rope or harness one handed while dangling like spider dinner .(although when i had to do that i used a 3" one handed opening lock knife on a string which would also be difficult to explain in the wrong place with the wrong copper )
that said treac's linky seems pretty useful to have at the back of one's mind for a just in case situation.
i wouldn't choose a stanley knife as a dispatch tool but it could be used if there was no viable kinder way available in a reasonable timescale.
||I doubt you'd be carrying a 5" fixed blade as you're unlikely to have good reason...|