If this "I can issue you an FPN" thing happens to you ands you're not happy about it always remember that as a Police officer I can only offer you the option of an FPN. You are perfectly entitled to refuse it and then the person in question has two options. A:/Let it go and send you on your way, and B:/ report you for process. If you are happy that you've done nothing wrong always opt for being reported. It'll shag the officer's day up if he decides to pursue it (report file etc., court date, statement of evidence, CPS advice etc. etc.) And also note whether or not he cautions you when he reports you. That'll likely be the last you hear of it. Especially if it's anything other than a Home Office plod, and less likely to know exactly what he's talking about.
Not trying to screw my fellow officers over, but I'm not a huge fan of people who dish FPN's out like they're confetti, and some do. FPN's are sometimes like meetings - the practical alternative to work!