Customs and Immigration are certainly security in most airports I frequent.
The word is used in the story below...
The UK Border Force (ie Customs and Immigration) is mentioned and they will do random stop and searches on inbound baggage and passengers as well as intelligence led searches. I wonder if there was some intelligence leading to the seizure in this case.
Outbound hold and hand baggage checks are done in the UK by the airport operator which may be using internal staff or approved contractors.