Agreed, "don't be in the situation" doesn't add a lot to the conversation. Nobbie48, thanks for the HTA section. Again, I wonder if not actually changing speed when doing it makes a difference. I have also done the "off the throttle move" indicating "look man, I'm not speeding up so pass". But that seems like it would be covered by the line as well. Taken in isolation, that line seems to really privilege speeders and tailgaters. I wonder how it works in practice.
A careful reading of the section that nobbie48 quoted will show that the charge is not for simply flashing your brake light, you must slow, stop, impede, cut off, or interfere with another motor vehicle. As drivers we know that exchanges like this happen over time and may not be easily explained if a police officer
only sees you braking in front of someone. It is very likely that the charge would not result in a conviction were it to go to trial and you explained the circumstances. That doesn't mean that you wouldn't be charged, however. It is up to the officer to determine if there are grounds to lay that charge at the roadside. I would encourage anyone in that position to have an honest conversation with the officer at that point.
Because the brake light flash is different than actually braking and slowing down, I would never charge someone under 172(1) for that. I would have to see braking action, like the front of the vehicle/forks going down, rider body position reacting to the braking, or something along those lines. To me, the brake light flash is a form of communication - and it's more polite than the hand signals that usually follow.
If it were me and I was able to move and let the person by, that would be my choice. I would probably save the brake light for the time when I couldn't go anywhere and really wanted to tell the other driver they are too close. Weaving in the lane may work as well but beware that other drivers may not know that you are allowed to move from left to right track within your lane, and could call in a nuisance traffic complaint about your riding.
The single best way to get the message across to a tailgating driver is to turn on the rear-facing emergency lights. It works rather well. In the rare circumstance that it doesn't, I arrange a brief meeting with the driver, and write it down for them.