Flyeyes is totally legal, they work with VOSA to get it right, well at least they did when I bought the stuff. My last car went thro her last 2 MOTs with it on her rear lights. It doesnt affect the amount of light coming out (much), its a honeycomb sticky film so light still gets out, but allows the user to tint their lights in various colours.
Pros -
legal tints
comes with (or used to) detailed leaflet explaining its all road legal, VOSA approved bla bla for any anal police peeps that pull u over
fun way to personalise ur car as available in many colours
cons -
for keen detailers polish can get stuck in the honeycomb bits and its a b1tch to get out out
also need the patience of a saint to apply and a good hardryer lol
noticed on my other car that on sharp-ish edges or deep hollows it was beginning to crack and stretch back after a year or so
But hey cost me £20 to tint both my other cars rear lights so happy days overall!