When I lived in Manchester my next door neighbour was a police officer. He told me that although it was technically illegal, you would only be asked to remove razorwire if somebody actually complained about it (so it depends what your neighbours are like).
If you have it well inside your fence boundary, i.e. not hanging over into public space, he also said that it was "highly unlikely" you would ever be prosecuted if somebody hurt themselves on it, as what were they doing on your property in the first place.
Of course I'm not suggesting anybody goes out and gets some
but he had never heard of a case where anyone had ever succesfully prosecuted someone for hurting themselves on razorwire whilst trespassing on their property