Having read the 'regulations' it seems that you are exempt from wearing a mask if you say you are, simple as that. Bit like declaring which gender you are, it's up to you, no questions asked, or rather none to be answered. I wondered if I might bother trying to secure an exemption on medical grounds (severe sinusitis in my case, confirmed by MRI scan in April), but there's nothing to be secured in the first place. If I say my health won't allow me to wear a facemask, then that's the end of the matter, in the context of shops anyway. I happen to know that it'd not be the case with hospital appointments, and quite rightly so imo. By the way, I shall endeavour to actually abide by the spirit of the regs for the time being, and actually put a mask on before I enter a shop because I get so sick of the pettymindedness directed towards us, and don't have the energy for TOO many arguments in one day.