I love it when people who haven't got the faintest idea what they are talking about give advice.
It's quite legal to carry a knife in the UK, within certain clearly defined legal parameters.
All sub 3" slipjoints are legal and require no justification for carrying. Further to that if you can show you have good reason for carrying a knife, or need it for work, you can carry any other sort of knife, with the exception of automatics, balisongs and push daggers. Self defence is not a good reason, just as it isn't in many parts of the US and the rest of the world.
Someone can only be stopped and searched in the UK if the police officer has a reasonable suspicion of an offence being committed, and that cannot be based on something as simple as dress, ethnic origins or even a previous record. The officer must have a real reason for the suspicion that will stand up in court.
Danzo