We've been doing this for years and there have been many posts in relation to this on here before. Any air gun F marked in a pentagon air pistol or rifle is included. Imports from the EU were specifically exempted in the regulations from face to face requirement or via an RFD. There is no need for specific letters, customs are not interested, and contacting them will just cause delays and confusion.
The only problem is awkward carriers who are not willing carry guns of any type other than for the manufacturers, its not a legal issue, its the carriers who seem to change their own rules at will, JG66, a list of carriers/couriers who will carry guns knowingly would be usefull if you've had any recently.
No problem Mike if its an F marked target pistol, just order it.
Good Shooting
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?