It's a right pig's ear bit of legislation: The government has to be seen to be doing something to reduce the rampant amount of knife crime but there also seems to be a political reluctance to enforce existing laws or to start stop & search for drug gangs etc. etc. and so effectively a side effect is taking it out on normal law abiding people.
There is already provision for a 5 year prison sentence for carrying a knife with a fixed blade, or with one over 3 inches long, or with a locking blade without a good reason in a public place. So most Swiss Army knives are OK except the slightly larger ones with locking blades. Most Leatherman multi-tools have lock blades which could be an issue, although it makes them safer to use for their intended purpose. Things like carrying a Stanley knife in your tool box to fit a carpet is probably OK, carrying the same to the pub is probably not, test on that is up to the courts though.
s15 seems to say you can still get a folding non-locking knife, e.g. Swiss Army, delivered from Amazon etc. but not something with the locking blade. A right pain for suppliers as well as those with a legitimate need.
All the dodgy people people would have to do is send someone over 21/25 to the local supermarket to pick up their tools of the trade (with some sort of ID if required, not necessarily their own!), whereas anyone wanting for example say a specific rescue knife e.g. cavers, sailors, divers etc. would have to jump through various hoops.
Also how do they know from a face to face transaction that only nice people can buy knives?