Hey, they've long since had bombs/drugs into the Alkham Valley covered
http://www.legislation.gov.uk/ukpga/1979/2/section/5
5
Time of importation, exportation, etc.
(1)
The provisions of this section shall have effect for the purposes of the customs and excise Acts.
(2)
Subject to subsections (3)subsections (2A) and (6) below, the time of importation of any goods shall be deemed to be—
(b)
where the goods are brought by air, the time when the aircraft carrying them
lands in the United Kingdom or the time when the
goods are unloaded in the United Kingdom,
whichever is the earlier;