No they can not confiscate an offensive weapon and let you go on your merry way.
This isn't primary school if the constable has grounds to suspect / believe your are in possesion of an offensive weapon then you should be arrested and it will be seized as evidence. If you give a lawful or good reason to be in possesion then they don't need to take any further action.
For information from the CPS website
The term 'offensive weapon' is defined as: "any article made or adapted for use to causing injury to the person, or intended by the person having it with him for such use".