Not exactly. I do have a card from B&H buses showing I'm exempt and it's hung on a sunflower lanyard that most major supermarkets recognise as identifying some one who is exempt or might require extra help due to a hidden condition. Doctors won't issue certificates to prove exemption.
I think a badge or something will save embarrassment on all sides, some **** at M & S Hollingbury was most upset at being denied access this morning, he promptly jumped into his Range which he parked over two bays and sped off, what a cock.