In UK, it is a requirement of the AIP under
ENR 1.6.2 - SSR OPERATING PROCEDURES, para. 2.1 (e), which reads:
2.1 Some Mode A codes are reserved internationally for special purposes and should be selected as follows:
(e) Code 2000. When entering United Kingdom airspace from an adjacent region where the operation of transponders has not been required; or by Mode S transponder equipped aircraft on the aerodrome surface when under tow, or parked and prior to selecting OFF or STDBY - unless otherwise instructed by ATC.
JD
