I was digging around the USPS website and found this:

https://pe.usps.com/text/imm/fh_014.htm

In the list of prohibitions:

Arms and parts of arms, or any component classified as munitions of war; imitations and antiques; paint-ball or toy guns; taser guns; air rifles, air pistols, components of firearms, and all other items similar to or resembling the foregoing.

So no more US airguns can be shipped to the UK from private individuals (classified adverts) and possibly businesses in the US! Certainly Bakers now only ship within the US whereas they used to ship worldwide. I believe something changed in this regard back in April 2016.