The "cruelty to animals" legislation you must be referring to is the Wild Mammals Protection Act 1996. The act is quite clear in the actions it forbids in terms of dispatching an animal and if you take the time to read it, it's far from "vague" or "a minefield".
The person who was prosecuted for the squirrel was not prosecuted for killing the squirrel but for treating inhumanely and causing unnecessary suffering. Vince, you obviously don't really know what you're talking about in this area.
Notwithstanding that, people who don't hold the relevant SGC or FAC are probably relocating foxes. The law isn't really enforced, and it's surprising how many people actually don't realise that this is in fact against the law. Yourself included.
As I said previously, shooting foxes is perfectly legal, there is no requirement for them to be sick or injured, they are a pest, they are not a protected species, it is perfectly legal to shoot them with a firearm providing that they do not suffer.