It looks to be illegal to use poison - government advice is a bit vague, the fox project (not a national charity) has some advice
Pesticides Act 1998 Ė although superseded and legislation now included under a variety of acts, pesticides and poisons must, by law, be used strictly in accordance with the instructions on the product. No poison may legally be used on foxes and anyone found to be in contravention Ė deliberately or by neglect Ė will certainly be prosecuted.
If you think a wildlife crime is being committed then you can report it to the Police Wildlife Crime Unit online or by calling 101. [good luck with that!]
You can also report wildlife crime anonymously to Crimestoppers, by calling calling 0800 555 111.
There was a telly programme, maybe the one show, that followed a paid trapper in London who was paid to catch foxes being a nuisance and then let them go elsewhere. The chap loved them. Another professional trapper shoots them, where push comes to shove, but don't see any argument for that. Found that in the Sun.....