I have always put a FFE bit of A4 inside and written 'INERT' all over the internal packaging of all the stuff I have sent but have often wondered why. Maybe just to put the EOD officer at ease a little.
I haven't had a problem yet, a few times when I was expecting a visit, like the time I was transfering my mortar collection from my garage to my house and a Police car drove past giving me VERY funny looks, and also the time where I thought it wise to transfer my Bren from my garage to my second floor flat, and got caught by my new nervous neighbour.
These things happen and I know that if I do get arrested (quite likely nowadays), I won't get charged, as I have nothing to hide and my collection is lawfull 100%. I am glad that the Police are doing their job well, as it does keep wrong-uns on their toes.
While a few people were concerned about this 'Police register', this is merely an intelligence system, searchable and inputable (maybe not a real word) by respective forces.
I am a serving copper and am fully aware of all the systems in place. If ANYTHING comes to light regarding ammo, guns, bombs, kitchen knives, deep water, sharp corners on kitchen drawers etc, an intelligence report is input on the local system by the local station/borough/constabulary, filed under address, name, vehicle registration or all of them. The information is looked at and actioned as appropriate.
Before the Police visit your address, this system is checked on all the above. If one comes back as something similar in the past, then a visit by local bobby would probably be appropriate. But No, you will not come up flashing yellow and marked as a collector (or you could be a nasty terrorist, get caught with something legitimate, then do what you want for years).
Don't worry about it. We are all legitimate and sensible collectors here and would like to keep it that way. In order for this to happen, some of us may end up getting checked out a little closer. We have nothing to hide so may as well just hand out as much info and detail about this wonderful hobby to those coppers who don't have the knowledge. They may also get the bug and join the fun.