The snag is that the exemption in S58(2) of the 1968 act for "antiques held as a curiosity or ornament" doesn't extend to replicas which yours almost certainly is. Therefore it is classed as a section 1 firearm and subject to the same controls as, say, a modern deer stalking or target rifle.
Crazy? yes, but then when did British firearms legislation last make sense?
Pay the dealer:
No more than approx $150 for a Martini-Henri long
No more than approx $180 for an Enfield-Snider long
You can expect to pay more for the carbines - but not too much more as most of them are certainly knocked off in Pakistan.
See the ATO and G1 Exports
As discussed previously they will laugh at you, but you want their signatures not their opinions.
Arrange to collect the paperwork
You should do this in plenty of time as I am sure you can imagine, there are plenty of people doing this and it is pretty low on their list of priorities.
There is no firerarms legislation associated with the imports of these weapon systems, but there is a requirement to inform the RAF that the items are inbound at your receiving airport. Speak to ATO they are usually a pretty good bunch - if not slightly odd with bits missing.
Don't forget to declare to customs at BZN! I brought antique firearm back from Kabul, certified to the hilt (antique and obselescent calibre), declared to customs in advance - who failed to show up...
They then turned up on my doorstep 3 days later, impounded firearm and tried to nick me for illegal importation et al. Much red faces when:
Customs discovered the 3 x messages left with various customs personna giving flight and firearm details;
RAFP (utterly utterly useless) discover that said RAF flight had not been manifested...
Result? One happy RHQ with addition to fireplace!