Mr Plod is a socially acceptable (if somewhat
cheeky) term for
policeman thoughout
Britain and former British colonies.
It is derived from the loveable (if somewhat dim) English bobby in the Noddy series of children's books, popular in the 1960s and 1970s.
These days, one would not be arrested for referring to a policeman as Mr Plod, although the officer being insulted might have a very stern look for you..