As far as I can tell it looks like the law is broken if your intent is to make the animal suffer.
Wild Mammals (Protection) Act 1996 1996 Chapter 3
An Act to make provision for the protection of wild mammals from certain cruel acts; and for connected purposes.
[29th February 1996]
BE IT ENACTED by the Queens' most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal, and Commons, in this present Parliament assembled, and by the authority of the same, as follows:-
Offences.
1. If, save as permitted by this Act, any person mutilates,