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Because knife law in the UK is partially down to the cops interpretation (ie you need to have a reason to be carrying any knife) I always assumed a penknife looks like the sort of thing you'd have in your pocket whereas a single blade might be more questionable even if it is within the acceptable limits.
There seems to be quite a bit of misinformation about the legality of various knives over the last few pages. It's been a while since I looked into it but I'm very sure the following summary is true.
If the police deem that you have an offensive weapon they can arrest you. Whether that be a machete, a screwdriver, or a metal-bodied pen. This will often be dictated by location and you - ie a screwdriver in your waist band at a football match vs one your tool belt on the way to a building site. But ultimately it's up to them to justify and you to contest.
If you want to carry a fixed or locking blade or one that's over 3" you need a reason.
If you want to carry a folding non locking blade under 3" you don't need a reason. (obviously the offensive weapon thing still applies)
If you're white, middle class and in a suit you can probably carry what you like (provided you have a reason) and in the event the police do try and charge you, make sure they know you'll contest it and promise to waste their time. If you're not, don't want the hassle of arguing, or can survive without a fixed/locking blade you'll have less problems with what people term as 'UK legal' ie non fixed, non locking, sub-3".
SAK / similar are even better as sacredheart points out it 'looks' like the sort of thing that's OK. A quick Google shows people have still had grief over 'UK legal knives' that look aggressive.
Because knife law in the UK is partially down to the cops interpretation (ie you need to have a reason to be carrying any knife) I always assumed a penknife looks like the sort of thing you'd have in your pocket whereas a single blade might be more questionable even if it is within the acceptable limits.