The Royal Mail website shows that posting packages to and from any country outside of the EU requires a CN22 customs declaration to be completed. i.e. packages sent from within the EU to Iceland, Norway and Liechtenstein are treated in exactly the same way as those sent to China, Australia, etc (and vice versa).
Knowingly importing an item which you've
purchased from a non-EU country where it is marked as a 'gift' is a criminal offence which, at the very least, will result in the seizure of the item.
See sections 2.1, 2.3, 2.4, 2.8 and 4.2 here:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&propertyType=document&_pageLabel=pageTravel_ShowContent&id=HMCE_CL_000014#P186_19100
Chris