The police have the right to demand production of all of the documented proofs that you would have for holding a driving licence, certificate of insurance or a test certificate (MOT)
They can only ask for these if you have been driving on a road or another public place (this includes motorway service stations, garage forecourts, car parks and industrial estates). If you are supervising a learner rider (or learner driver) you can be required to produce a driving licence. Any police officer of any rank can require sight of your driving licence and traffic wardens also have the same power.