Things cyclists should know … POV driver who's incapable of cycling

As has been said above, window tint is not part of the MOT so unless you have used a photometer under controlled circumstances and can prove that the window lets in at 70% of the light from the test source then no, your vehicle isn't road legal at all.

You need to re-read the post you're relying on for information, since you seem to have gleaned only misinformation from it. The 70% applies to windows to the side of the driver, not any side or rear window behind him.

Meanwhile, this option is going to add <£1 per month to my payments, money well spent

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