Lots of people carry and use knives in the course of their work and this is fine.
However a knife is also a weapon that, if used in anger, could injure or kill. It is an offence to have with you a knife which has a blade, or is sharply pointed. This includes a pocket knife with a cutting edge more than 7.62 cm or 3 inches.
If you have a knife in a public place you will be arrested.