No GP's are not going to provide a stamped letter. Most of the time those asking are not exempt anyway, so they are told no letter. The ones that are truly exempt tend not to be out in the middle of Asda anyway.
I was putting my mask on in the doorway of Morrisons last week, three young lads, all mid 20's walked past and each one shouted asthma as they walked in. The security guard said please can you wear masks, one turned round and put his finger up and all three carried on into the store laughing