depends on what you flashed ?
years ago, I offered a hand gesture to what turned out to be an unmarked police car, after the tw@t did an emergency stop and U turn right in front of me, causing me to do an emergency stop. Bugger me if he didn't do the same thing 100 yards behind me to come back and pull me over.
I was informed that we the public were not allowed to make obscene hand gestures to her majesty's finest, and that he was about to report me for the consideration of prosecution. To which my reply was "If you drive an unmarked car, it's whole purpose to remain undetected, like you just did, far below the standard of any police driver, how am I to know that you are indeed a police officer when I am offering a hand gesture ?"
I was then informed that we the public were not allowed to make obscene hand gestures to anyone and that he was about to report me for the consideration of a public order offence. He then asked me to turn out the contents of the back of the van, which I refused, and invited him to help himself if he so wished.
I once again expressed the opinion that the gesture was born purely out of exasperation of his poor driving, and that were I indeed taken to court over the matter, I would as a matter of course instruct my solicitor to retrieve and make public the CCTV footage of the incident, clearly showing his poor driving.
At that point, the young lad (well younger than me), received a very quiet (I never heard it) call on his radio, and had to leave on an emergency. He did let me know that he would be telling his colleagues to be keeping a special eye out for my van in future.
Which was funny really, because I was in a hire van, with the name of the very well known hire company down the side.