"Plain clothes Police are operating in this area" which means presumably extravagantly dressed Police are operating everywhere else.
Which leads me to wonder why a policeman out of uniform is described as wearing plain clothes? I guess that the original Victorian blue uniform with the strange flower pot helmet (described by policemen today, I believe, as a tit* because of its because of it curved shape giving way to a metal 'nipple' at the top) was purposely outrageous so that the officer could be easily seen on a crowded street. We are so used to the hat these days we no longer see its strangeness.
It may be that the police actually call their hat a tit because it is short for Tit for Tat which is Cockney rhyming slang for hat. (This is more usually shortened to titfer.) Maybe that is how the helmet first got its name and the rude connotation was mere happenchance.
As everyone knows fewer and fewer policemen are wearing the helmet; For one thing no car is tall enough. It is more usual to see them in caps or even baseball caps ("What," says John Bull, "Not even cricketing caps but foriegn rounders ones").
I remember a friend in the force telling me that at the first sign of trouble; whether it was to chase someone or calm down a fight the first thing a new policeman learns to do is to get rid of the helmet because it only gets in the way.