I personally don’t like to sneeze with a mask on!!! Then there’s snot in your mask all day?
I want to sneeze into a tissue and put it in the bin. If I had a cold I wouldn’t wear a mask, I would just take the usual precautions - hand washing, not getting too close to people, tissues… it’s just common courtesy.
How, though? You still need to buy food and essentials, even when you're feeling under the weather. You turn up at the door of Asda, the mask-monitor/security person asks you to wear a mask, offering you one if you don't have one, and you say "No, I'd rather not wear a mask, thank you, as I have a bad cold"?
A lot of people are running scared of any illness right now and they have no idea whether you have Covid or not, irrespective of the cold (although they do realise that frequent sneezing will hugely help to spread whatever you might have).
Even if the doorperson does somehow allow you in without a mask or exemption, you'll get nasty stares and probably also abuse from all the customers. Genuinely, how would that work?