This is not specific to LHR, it is (and has been for as long as I can remember) a requirement at all UK airfields. It is in the UK AIP and listed as an ICAO difference in AERAD/LIDO/JEPP documents.
From LIDO CRAR "Set Mode A Code 2000:
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When entering United Kingdom airspace from an adjacent region where the operation of transponders has not been required.
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By Mode S transponder equipped aircraft on the aerodrome surface when under tow, or parked and prior to selecting OFF or STANDBY - unless otherwise instructed by ATC."