asked Her Majesty's Government:

What methods are used by United Kingdom security and immigration personnel at airports to identify themselves to entering passengers when carrying out sudden spot checks outsider the normal immigration queues. [HL1185]

Immigration officers undertake document checks at the gates of arriving aircraft on an intelligence-led basis. Officers wear high-visibility tabards/jackets to identify them as a member of the United Kingdom Immigration Service and ensure their warrant cards are visible during the document check.

Special Branch “ports” officers usually stand at a podium or point marked out as “Police Control”. At the point where they intervene in passengers' movement, they would announce themselves as a “police officer” (showing a warrant card, if necessary).