The Law
Section 1 of the Prevention of Crime Act 1953 prohibits the possession in any public place of an offensive weapon without lawful authority or excuse. (Archbold, 24.106a)
Definition of an offensive weapon. (Archbold, 24.115)
'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. The courts have been reluctant to find many weapons as falling within the first limb of the definition and reliance should usually be placed upon the second. On that basis it must be shown that the defendant intended to use the article for causing injury.
Lord Lane, CJ. in R.-v-. Simpson (C), (78 Cr.App.R.115), identified three categories of offensive weapons: those made for causing injury to the person, i.e. offensive per se; those adapted for such a purpose; and those not so made or adapted, but carried with the intention of causing injury to the person.
In the first two categories, the prosecution do not have to prove that the defendant had the weapon with him for the purpose of inflicting injury: if the jury are sure that the weapon is offensive per se ...
Offensive per se
For the caselaw on instances of weapons considered to be offensive per se (Archbold 24-116).
Section 139 of the Criminal Justice Act 1988 prohibits the possession in a public place of any article which has a blade or is sharply pointed, (including a folding pocket knife if the cutting edge of its blade exceeds 7.62cm/3 inches). (Archbold, 24.125)
Both offences under section 1 of the Prevention of Crime Act 1953 and section 139 of the Criminal Justice Act 1988 are triable either-way. The offence of having an offensive weapon carries a maximum sentence of four years imprisonment...
A dangerous weapon includes an offensive weapon as specified by the Criminal Justice Act 1988 (Offensive Weapons) Order 1988, as amended in 2002, or knife, bladed or pointed article.
http://www.cps.gov.uk/legal/section12/chapter_c.html#06