I don't particularly care what you do, as long as you are mature enough to believe you are doing it safely. I'm not perfect either and in certain scenarios, one can exceed the speed limit responsibly.
I think if somebody is going a whopping THREE KILOMETRES under the speed limit, then they really don't need to be tailgated and flashed. Ever. If they don't feel safe going any faster in their vehicle, then other road users should be grateful of the fact they are driving within their limits and not endangering them. I believe there are no dangerous vehicles, only dangerous drivers. A dangerous driver in an unsuitable vehicle for their attitude is the worst combination. Most people have a good reason for doing something. They could be driving slow for instance, because they are on their way to get their bargain basement tyres fitted, for instance. Because they are on their way to get their brakes fixed.
In short, don't assume you own the road and everybody is purposely out to disrupt the traffic. Some people are pretty hopeless, no denying that, but I'm certain that people who you honk and flash, there are some that really don't deserve to be bullied on the road, and you probably made their day a lot worse.
Especially for going 5% under the limit, which is a limit, not a minimum.