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Some machetes are collectible because they've been standard military equipment for ages, not as weapons but for clearing a path in jungle/forest. https://www.thebushcraftstore.co.uk/martindale-golok-british-army-machete---the-genuine-one-78-p.asp They're also widely used for camping and gardening. And every butcher uses them. https://www.s-staniforth.co.uk/specialist-woods/278-butchers-knife.html
Some zombie knives are collectible because they come under the heading of film/game memorabilia. They're usually very cheap, made of rubbish metal, but I suppose if you have ill intent they represent the best value, because you use them once, throw them away and you've only lost 30 quid. https://weaponsgalore.com/product-category/swords/end-of-days/
And if you're going to talk about machetes and zombie knives, it's logical to include bayonets, swords, axes, meat cleavers, scythes etc.
As you say there's already an age check. Some couriers provide this as an extra service for online sales....the recipient of the parcel has to show ID.
Here's a fairly collectible RAF machete. I've got a similar one but it's cheap and tatty https://theantiquescentreyork.co.uk/collections/gifts-under-500/products/a-world-war-11-royal-air-force-folding-machete
There isn’t really a valid reason to carry a zombie knife or machete, they are not really collectible, and in the urban jungle, likely only used for intimidation and violence.
Kitchen knives are legitimate kitchen tools, and you can’t really ban the sale of those. There is already an age check on blades of any type.