2000 is the IFR conspicuity code in the same way as 7000 is the VFR conspicuity code. (For the Americans it is used this side of the pond the way you guys and gals use 1200.)
The nearest thing to legislation I can find a link for:
https://www.caa.co.uk/docs/33/SafetyNotice201107.pdf
Special Purpose Mode A Codes
Some Mode A codes are reserved internationally for special purposes and should be selected as follows:
Code 7700. To indicate an emergency condition, this code should be selected as soon as is practicable after declaring an emergency situation, and having due regard for the over-riding importance of controlling aircraft and containing the emergency. However, if the aircraft is already transmitting a discrete code and receiving an air traffic service, that code may be retained at the discretion of either the pilot or the controller;
Code 7600. To indicate a radio failure;
Code 7500. To indicate unlawful interference with the planned operation of a flight, unless circumstances warrant the use of Code 7700;
Code 1000. To indicate an aircraft conducting IFR flight as GAT, where the downlinked aircraft identification is validated as matching the aircraft identification entered in the flight plan;
Code 2000. When entering United Kingdom airspace from an adjacent region where the operation of transponders has not been required;
Code 7007. This code is allocated to aircraft engaged on airborne observation flights under the terms of the Treaty on Open Skies. Flight Priority Category B status has been granted for such flights and details will be published by NOTAM.
Note: Mode C should be operated with all of the above codes.