Here is a page from the UK Post Office website. I draw you attention to the box at the bottom of the page Weapons and Ammunition.
“inert items will not be permitted” to paraphrase They seem to be referring to grenades and artillery shells but it will lead to “mission creep” mark my words. Whats a “shell” in common parlance? Its the first time that I am aware of inert items ever being mentioned in any official context
I don’t know anymore than you can read on the page. The big concern on my part is the undefined use of the word components elsewhere in the report. Since only face to face purchase of live ammunition is allowed this has had a bad effect on owners of the less than common calibres. UK is not a big country but there are still some pretty remote areas and gun shops few and far between.
Does components mean scopes? plastic mags? bipods? Many of our existing retailers are just about hanging on by their fingernails selling aftermarket triggers and the like by mail order