The requirement to select 2000 prior to standby in the UK is found in AIP ENR Section 1.6 para 2.2.1(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