Sorry to dig up an old thread, I was googling to find an answer and this was the top result.
I’ve spoken to Motability today and they say the only way you can have tinted windows is if they come as standard/are a paid option at the time of ordering or you supply Motability with a Doctors letter explaining why you need them. You cannot under any circumstances have them fitted otherwise.
I thought there page on it here https://news.motability.co.uk/motoring/privacy-glass-explained
indicated that you could with permission but apparently not.