Legally its no issue, no duty or import restriction if its an EU legal F marked gun, its simply posted, if accurately described the UK end is no issue.
The problem is that many (most) of the carriers since the terrorist events will not carry any gun, air or otherwise, this is due to restrictions on ships and aircraft which put expensive limitations and obligations on them to declare and separate, so they have chosen to not carry any guns, or charge very high prices to courier.
Some people are "economic "on how they describe it and post it as sports equipment or something, but a dealer I know well in Germany says its getting very strict and they get post returned from their end before it even reaches the UK. He does it by the book and still gets stuff returned. Some gets through, some not!
Best of Luck!
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?