Check out:

http://www.met.police.uk/firearms-enquiries/airguns1.htm

Its an offence to give (including sell) an airgun to anyone below 14, not 17 (currently).

You can (currently) carry an airgun (not a pistol) in public in a paper bag if you want as long as its not loaded or cocked for whatever reason. Note from the MET site that the proposed laws will change this significantly and you will be a criminal.
Lets all take a moment to lament the fact that Britain used to win tons of Olympic Golds for shooting but now we're regulated (and relegated) to the minor league...ordinarily, responsible shooters aren't responsible for criminal shootings - that's the scum holding illegal weapons, but we all suffer.
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In the end, all you have left is style...