I know a few Police Officers and the majority would prefer to use their discretion, but they can't. They give two reasons for this;
First, the fear of having the nice, respectable looking gent with the penknife walk away from them, down the road to his estranged wife's house where he proceeds to kill her and then himself. The Officer will then be held responsible by the media and the political elements within their force.
Second, despite the protestations of the politicians and senior management within the forces they do have a target culture and must justify their existance by recording detections. For example, and officer based in a London airport was called to attend a person who had a lock knife in their hand luggage. No real reason, just that they had forgotten it was in a side pocket (as many of us here have also done). He would have been happy to allow the knife to have been disposed of or posted back to the owner's home address. Unfortunately as the report had gone through channels and was recorded it had to be fully processed and now the knife owner has a criminal record.
So as others have said, don't blame the Police (in most cases). It's the blame and media driven culture we have allowed to develop in this counrty which is responsible.
Back to the original question. It is legal to carry almost any knife with a good reason. If you are worried, just as many have said here, keep them out of site and don't be a plonker and bring attention to yourself. That way you won't have to have a talk with a Police Officer in the first place!