We have section 58 for obsolete caliber Jim. Our government (Home Office) publishes a list of obsolete calibers and guns on it.These are "off ticket".

section 7.1 covers collectables allowed at home these are held on a Firearms Certificate. As you say ammunition cannot be held for them but...... if you have a .38 revolver on 7.1 and perhaps an underlever rifle in .38 under section 1 you would have the ammo at home for the rifle (it would not of course be used in the revolver).

section 7.3 allows pistols to be held at Herritage sites but we have to assemble and store its ammo at the site.