You can buy 14, 24 and 32 bore shotguns in the UK as antiques without holding a Shotgun Certificate unless you intend to use them in which case you need one.

There are obsolete calibres for guns, but no such thing as “antique ammunition” (except cartridges that do not contain their own means of ignition).

So to buy 32 bore etc cartridges you still need a shotgun certificate.

1/2 ounce in a 32 bore should in theory pattern better than a .410 ( shorter shot column) but it probably varies from gun to gun and cartridge to cartridge.