It's very grey area. It's illegal to release "vermin" but there isn't a real legal definition of what are classed as vermin...
8 Oct 2003 : Column WA60 Vermin
Lord Selsdon asked Her Majesty's Government:
Which mammals and other animals are classified as "vermin".[HL4559]
Lord Whitty: There is no definition of the term "vermin" in UK law. In such a situation the Oxford Dictionary definition should be applied.
The Oxford Dictionary defines "vermin" as "Animals of a noxious or objectionable kind. Originally applied to reptiles, stealthy, or slinky animals, and various wild beasts; now, excluding in US and Australia, almost entirely restricted to those animals or birds which prey upon preserved game . . ."
The Small Ground Vermin Traps Order 1958 and the various Spring Traps Approval Orders, refer to "small ground vermin". Neither the orders nor the Pests Act 1954, under which they are made, define this term or provide an exclusive list of species. However, the following animals are listed under various orders: moles, grey squirrels, rabbits, mink, stoats, weasels, rabbits, rats, and mice.
Traps approved under the Spring Traps Approval Order 1995 do not apply to small ground vermin listed in Schedules 5 and 6 to the Wildlife and Countryside Act 1981. This means that red squirrels, dormice, water voles, shrews, hedgehogs, polecats and a number of other species are excluded.
http://www.publications.parliament.uk/pa/ld200203/ldhansrd/vo031008/text / 31008w02.htm