In a home a knife can be considered a tool, mainly because it is one. Once on the street (in the UK), it becomes a potential weapon in the eyes of the law. But so does a screwdriver or yes even a pair of scissors.
Any hard or sharp object with no other use in it's then current surroundings, can it seems, be considered a weapon. So yeah you can carry a 4" long fixed blade but only if you're out hunting or doing some wilderness survival exercise or something. A cue ball isn't something you really want to get caught with either unless there's a pool talbe nearby.
I've never had any problems with the police regarding knives but I do carry a locking blade at all times (two of them actually as I've got a leatherman wave that I never leave the house without except to go running). I use my knife or knives (depending on what I've got with me) a number of times during the day and I would no more like to be without a knife than without my shoes.
I could be in a crowded shopping centre where the "law" would say I have no genuine reason to carry a knife. However, I do as I'm sure do we all ocassionally, find myself in such a place and needing a knife. Infact a number of times I have been with people I know (at work or elsewhere) and they have needed a knife, so I have brought mine out to let them use it only to see the shocked look on their face, as if to say "you carry that around with you?!!!". I could understand if it were a big Rambo-sized knife or something that would scare the shite out of Crocodile Dundee, but it never is. Still they're concerned and I can't understand why, or why *they* don't carry one themselves.
Half of the worry people have about knives is that they don't seem to understand that some of us who are practical people, find them useful. Instead they use their teeth to take a tag off a new purchase or go into a nearby shop to borrow a pair of scissors to open a package.
The other part of the problem is that with the increase in violent crimes (I would guess there's an increase, otherwise it's just being reported more than it used to be) the younger generation are scared, though most won't admit it (cos they're all so hard. Yeah right!). In the UK young males between I think 14 and 22 or something like that are the group most likely to be mugged etc... I'm in that group and I'm a bit concerned, I don't go out late particularly and I don't go to areas that are known to be dangerous (these are the reason why that group is in most danger, they go where sensible people wouldn't because they think they *should* be able to handle it) but I know the dangers are out there. Because others in this age group do go to these places and take these chances, some feel they should carry protection. I once knew a guy at college that came in one day with a hammer in his bag claiming that some bad sh*t was going to go down. Some carry these potentially "offensive weapons" to hurt or threaten others, but most carry them because they are scared of the first type of people.
The UK is suffering from an increase in the number of firearms, mainly handguns, and mainly in the hands of people with bad intentions. Obviously due to this combined ith the above, the government are rather concerned, as am I and many others no doubt. Having said this though to target people for simply carrying a knife, and putting then in prison for 2 years is rediculous. There has to be some way to differentiate between those who are likely to be carrying knives as weapons, and those who use them as tools.
I rarely carry a knife with the thought of SD in mind because if the guy is quick enough I won't have time to get to it and if he's not quick enough then I could more than likely put him on his arse without needing a weapon. And more to the point I would much rather run away or throw a brick because it means I'm less likely to get hurt myself. However I do worry about being searched (for whatever reason) and found with a knife to which my only explanation for carrying it would be, "well I never know when I'm gonna need it", but I never do know.