Having an Indian curry to create enough content to fill a jiffy bag and then posting the poo to your debt collector / baliff / whoever issued you a fine.
Dave: Yo man, did you ever pay that Ulez fine you got?
Steve: Yea man, I sent em a Big Narstie in the post
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.