So is it not endangering if you are in a position where you could get a pellet over the highway? If your pellet could ricochet into vehicles or a cyclist or pedestrian then you are surely subject to this law even if you had not intended it? Seriously, please clarify this as I read it that the risk is related to the offence as much as it happening. I am probably wrong and know that it is unlikely to be enforced even if it did apply.
On a more personal opinion I do question the use of firearms in private gardens when there are less offensive options. You can get legal and humane traps for most animals in this country and in an enclosed place then perhaps they are better than an air rifle or other weapon. I do reckon professionals you would bring in would not use an air rifle when there are traps for the job in a garden.
Many years ago I was in a friend's garden and we were "having a go with" (aka playing with) her air rifle
we set up some bottles on bricks and took pot shots at them
After a while we noticed that some of the missed pellets had punched a hole through the fence behind - which seemed sub-optimal
so we put an old metal bin lid behind the target to block them
you know the ones - curved metal lid with a handle on the top
I took a shot and it missed the bottle
next thing was a crack as the pellet hit the wall just by my ear
It had hit the bottom of the lid - must have whizzed round the curve and directly back towards me!!!!
pellets don;t always go where you expect!!