One of the traditional South American methods for creating a blowgun was to entomb a grub in the pith of a long branch, completely sealing the end. It's only means of escape is to eat its way through the full length of pith and thence to freedom. The darts were/are made from slivers of bamboo with fluffy plant fibres (remarkably similar to cottom wool) wrapped around the blunt end to act as the air-seal. The business end of the dart was/is repeatedly dipped in a poison made from the exudate of the inner bark of various trees of the Virola genus. The poison doesn't kill the prey, but temporarily stupifies it.
Also, in the UK, they are only illegal to manufacture, sell, hire, give away, expose or possess for the purpose of selling or hiring. That does not include simple posession. It would of course be illegal to use one for hunting (as it's illegal to use most things which aren't a rifle, shotgun or air rifle for hunting the appropriate quarry).
Section 1.(k) - http://www.opsi.gov.uk/si/si1988/Uksi_19882019_en_2.htm
So I suppose if you owned one before 1988, you can keep it at home. Although I'm not entirely sure what "expose" means. Would that include showing a friend or family member in your own home? Anyway, it hardly matters as I doubt many of us just happen to have a blowpipe that's been hanging around the house for the past 18 years.
You are however free to buy, sell, posess and
expose antique blowpipes, assuming you could find one.
Pappa