Illegal 'without good reason'. Reasons could be outdoor activities; fishing, shooting. Or for work etc. Context is everything, unless you have a copper who just wants to make your life miserable.
2 Things I have in my pocket / on my belt right now: swiss army penknife, leatherman super tool300.
The SAK goes everywhere, it's never not in my pocket, be it at the office of on a night out in town.
The ST300 is prety much the same but for one thing - the locking blades make it a 'fixed blade' in the eyes of the law and therefore restricted 'without good reason'. Because of this, I'll leave it at home if I'm going out on the town. But If I'm just out and about I'll have it with me more often than not. The pliers, screwdrivers, and the locking blade have all been useful as tools. All the same I'll not be needing it down the pub in all likelihood. So it stays at home so plod won't ruin my night out.
I don't need a 'good reason' (excuse) to have the SAK - non locking blades less than 3" in length.
I've always believed in taking responsibility for your own actions. It's a shame thugs have made the rest of us have to jump through hoops.