I know you can get a kit that connects to your 3rd break light and flashes when you hit the breaks and is legal in the UK
Probably a stupid question so please don't laugh lol
Would you be aloud by law to have all 3 break lights flash then stay on the only reason I ask is I seen a car in Swansea today with them all done and looked really good I was about 3 card behind and it really count my attention iv seen this car a few times as I deliver for yodel in the town centre

 

Not sure it helps you but my old astra vxr would flash the brake lights of you broke hard! Think the idea is it lets people know its an emergency you are braking for! All 3 lights would flash!

 

I had a volvo s40 r design that flashed the 4 ways if you hit the brakes hard... seemed odd to me,,...