Right then, pre-1898, that simplifies things sort of. The way I understand this is that ParcelForce is the Royal Mail version of our USPS Parcel Post, and that the two 'official' mail services work seamlessly together. ParcelForce is the way my pre-1898 gun was sent and when it got to the States it went right into the USPS mail to me. Now, however, by the original post it appears that ParcelForce of the Royal Mail is no longer accepting firearms. Dang.

I don't know what shipment arrangements besides air freight might be available in the UK for this. John and Rhonda Foster do this kind of thing all the time and I'm sure they will be scrambling to discover a new method. Short term it looks like you might be stuck with air freight.