It is a tool in a tool kit therefore you will be fine. It's purpose is to for general maintenance and repair of the vehicle and whatever else you would use the other tools for. But you couldnt keep a machete under the driver's seat or have a bowie in the door pocket (unless you are driving to a camping trip of course and even then its best kept in the boot, lol). The UK law has a lot of flexibility for common sense. Despite what some like to report there isnt a huge hunt with police locking up otherwise completely lawful citizens driving around with Leathermans in the car.