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.....
Guys, the police has answered my question!
"These types of lights are illegal for use on normal motor vehicles. Regulation 13 of the Road Vehicles Lighting Regulations 1989 states that flashing red lights to the rear of a motor vehicle may only be fitted to vehicles used for police purposes (except rear red flashing lights on pedal cycles) so whilst there is no law preventing the sale of these items it is against the law to use them on your private vehicle. "
Just wondering how Mercedes will cope with these regulations?