I have this text off the AA about putting products on your lights
Products on the lens or light source that obviously reduce the light's intensity or change its colour will become a reason for failure – applies to front/rear position lamps, registration plate lamps, stop lamps, rear fog and direction indicators,
Fly eyes reduces light intensity, I actually took mine off as it was reducing the beam so much (it is very noticeable). Obviously the above is open to interpretation, and some garages will fail you on it some wont. Appreciate their website says they adhere to the guidelines but you click the links for the FAQs on this subject and there is nothing there
Up to you really, you can always rip it off at MoT time if they fail you, but do bear in mind it does significantly reduce the beam.