Yes, that is essentially correct. Most bladed weapons are classed as offensive weapons (including lock-knives etc), with the specific exclusion of folding pocket knives below 3" in length. As long as you have a reasonable excuse for carying the knife and you aren't waving it about in public or scaring people with it then you "should" be OK.
However, note that almost anything can be deemed to construe an offensive weapon based on the policeman's subjective opinion of whether you are likely to be a threat to public order by your use of the object. A policeman could still arrest you for having even a small pocket knife (or virtually any other object) if he has reason to suspect you of bad intentions.
@nicholas.lee: For the benefit of the few of us who are NOT barristers, can you place this in some context? I surmise that the meaning is that in UK you can carry only folding pocket knives with a blade length less than 3 inches; any other type of knife regardless of blade length is forbidden (???). Is this correct, and the entire implied content of your post?
In the UK, Sections 139 and 139A of the Criminal Justice Act 1988 apply to any article which has a blade or point except a folding pocketknife unless the cutting edge of its blade exceeds 7.62 centimetres (3 inches).
However, if a policeman has any reason to suspect you intend to do harm with your knife then you may still be required to prove otherwise in court!
The maximum penalty for these offences is a prison sentence of four years, or a fine, or both.
For this reason I like the "Leatherman Keyring Tool Squirt PS4", it is a great little multi-tool for fixing things, and more importantly (from a legal perspective) nobody is ever going to to feel threatened by it.