The polecat is listed in schedule 6 of the 1981 Wildlife & Countryside Act, and Schedule 3 of the 1994 Conservation Regulations.
Under these laws, certain methods of taking wild polecats are prohibited.
These Acts prohibit the deliberate or intentional setting of traps, snares and so on for polecats without a licence. Where traps are set for other (pest) species, the 1981 Wildlife & Countryside Act requires that reasonable precautions to prevent injury thereby to any wild animals included in Schedule 6. Regulation 41 of the 1994 Conservation Regulations prohibits the use of traps that are non-selective according to their principle or conditions of use for taking or killing of protected animals, including the polecat.
In addition, the 1995 Spring Traps Approval Order specifically excludes the use of spring traps for the capture of any species listed in Schedules 5 and 6 of the 1994 Wildlife & Countryside Act.
It is legal to shoot polecats with appropriate firearms, and there appears to be no lawful reason why polecats (harmed or otherwise) caught in traps cannot be shot or kept in captivity. All in all, a confusing state of affairs.