The law about carrying knives in a public place is governed in England by the Criminal Justice Act 1988. This puts the responsibility on the individual to show they have a ‘lawful authority or reasonable excuse for carrying it’.
There is an exception made for folding pocket knives (pen knives) with a cutting edge of less than 3 inches (75 mm) which does not lock.
THE GENERAL ADVICE FOR FORAYERS IS THAT IF YOU BRING A KNIFE :
• Ensure it has a small blade (absolute maximum 3 inches)
• Preferably a folding blade (such as a penknife)
• Keep it well hidden but accessible only when needed during the foray
• Keep the knife well out of reach of children
• If these sensible rules are adhered to it is perfectly acceptable to bring a knife to a foray. In fact, it can be a very useful tool.
A more detailed description of the laws concerning knives can be found here.