I reckon the Animal Welfare Act should cover it.
It covers protected animals which are commonly domesticated in the British Islands, under the control of a person on a permanent or temporary basis and it’s not living in a wild state.
There is an offence of administrating a poison…..
Administration of poisons etc.
(1)A person commits an offence if, without lawful authority or reasonable excuse, he—
(a)administers any poisonous or injurious drug or substance to a protected animal, knowing it to be poisonous or injurious, or
(b)causes any poisonous or injurious drug or substance to be taken by a protected animal, knowing it to be poisonous or injurious.