Last i checked, its still a greyish area.
DVLA / VOSA hadnt tested, therefor approved any reflective material for use.
You need to read road traffic laws, not mot really, as they are different. I can pass a car which has blue flashing lights on the roof, doesnt mean the police wont do you for it.
My worst story...is getting a fine for running pressed plates. Legal font, spacing, background, size the only thing which wasnt allowed, was they had the French 'F' under the Euro Flag (ring of stars) instead of 'GB'
I got pulled 3 times with these on, never mentioned.
Then one Monday morning, in winter, i come out at like 6 am, theres a ticket on my screen for the plates. Left at like 3 am, while car was parked in a quiet new build kind of estate area, so no through roads. With cars parked in front and behind. So youd struggle to see the 'F' !