From what I've seen, UK Customs - damn their extortionate hides

- are *generally* not bad. Yes, stuff does go missing occasionally, no point denying it - but in 12 years of ordering stuff from the US online I've not lost anything.
I'd be more worried about them billing me - and Parcel Force adding their tithe on as a "handling charge"... It's a really bad idea to mislabel the value, too - sometimes they want receipts.
Added - there have been attempts at trying to claim one or two knives are "illegal" by overzealous officious individuals, but all of the incidents I know about they've backed down. We have weird legislation in some areas - the following are illegal to purchase, make (hmph!), hire etc. etc., so would be confiscated if found (list limited to just sharp & pointy):
Automatics (although strangely, not assisted openers. Yet)
Balisong or butterfly knife
Push dagger
Shuriken, shaken, or death star
Handclaw
Footclaw
Swordstick
Kusari gama (rope, cord, wire or chain fastened at one end to a sickle)
Kyoketsu shoge (rope, cord, wire or chain fastened at one end to a hooked knife)
Belt buckle knife
Disguised knife
Stealth knife
HTH
Peter