Old topic but I came looking as mines had some leds out for 3 years now,
MOT and legalities from what I was told,
in the 1st year there was a couple not working and it was a pass with advisory only.
next MOT as it was now less than 50% working it was a fail. however if I pull the power from it so it doesn't work at all it is a pass as it is not a legal requirement to have a centre high brake light.
SO essentially if less than 50% either replace or pull the power (can't remember if I pulled the power or it was an single fuse as it was almost 2 years ago now)
Hope it helps someone with the MOT legal points.