Depends. I managed to import £2200 worth of Anschutz 1913 Supermatch in without problems (paid £1800 for it from a lovely German dealer. Result!).
On the other hand, many have been sent bills from HMRC. There are particular problems with airguns as they mistake them for airsoft guns - i.e. RIFs. You have to patiently explain that it's a real firearm and not an imitation firearm, and therefore legal, and whilst they're busy impounding it, they figure they might sting you for tax at the same time.
If the paperwork is in order (i.e. export permits from Germany, etc) and it is clearly addressed to an RFD, it should sail through.
If they do ask for money, I guess it will be VAT (since that is what you are avoiding by importing stuff), so 15% on the value.