According to the official BAA site for Stansted, stopping on the forecourt is not permitted.Drivers are to wait in the designated area (which, from memory, is still free for 15 minutes or so). .
It's quite likely that there are recent regulations, made as by-laws for the good governance and control of London Stansted Airport, just as there are regulations for Heathrow. These are made under act of Parliament ( I had to research the Heathrow ones once) giving the power to make the regulations to the specified government minister, which regulations are, in fact ,drafted by some civil servant and approved by the minister, whereupon they become law.As such they are enforceable through the local magistrates' court, as offences.
That said, if this man had no uniform and did not produce any proof of authority and entitlement, it was almost certainly a scam. If it wasn't a scam and the regulations give a power to demand an instant fine (or any penalty), you'll probably find an official letter is sent to the registered keeper of the vehicle.In any event, you have what seems the perfect answer viz. that their official produced no i.d or proof of authority and behaved in a manner and made demands wholly out of keeping with his office.
Why not ring Stansted secirity and ask? You may ask as a concerned intending traveller who has been told of this practice, and who wishes to know the position and law .If it's a scam Security ought to know about it and take action.