If being held as a curiosity or ornament they cannot be shot at all, unless the relevant FAC/SGC applies if I have read the guidance right. As soon as you show any intent to fire them they become subject to all the relevant firearms controls.
Keeping them on the mantelpiece as a collectors piece is ok but then to get some powder and load it they become a firearm. Unless I've missed something on the guidance.
Yes! Date controls in the US: obsoleteness of caliber controls in the UK (for cartridge guns).
If your Martini Henry rifle were made in 1899 or later, it would have been readily importable into the UK: but NOT readily exportable from the US.
The US laws aren't-always more lenient than the UK's.
UBC's Police Pistol Manager
"Nasty, noisy things, revolvers, Count. Better stick to air-guns." Sherlock Holmes, The Adventure of the Mazarin Stone