The rules are the rear brake lights must stay on, but that only applies to the ones each side of the rear which all cars have by law.
Not all cars have the central high brake light so I cannot see any reason why that one cannot flash.
For the correct answer to your question ask it here.....
Thanks for the link. Have posted my question there but may still take some time until and if they will answer it.
Have found a review to the new B-Class Mercedes:
Active safety: flashing brake lights as warning to other motorists
The flashing brake lights are triggered automatically if the driver initiates emergency braking when travelling at a speed of 30 mph or more. And if the B-Class is braked to a stop from a speed of over 45 mph, the hazard warning lights will come on too.
I guess should be legal, at least have some arguments in case I will be stopped for using it