Maybe get a pack of glucose tablets to sit in your car glove box, one or two of them should buy you enough time to park officially before going into the shop. 3 or 4 gluco-tabs should solve the hypo without need for the shop allowing you to stay with the car until feeling better. They usually have a very long shelf life, and don’t get affected by hot or cold.
Added bonus, if you stay with the car during a hypo and a traffic warden walks up, you wind down the window explain the situation, tell him “I’m diabetic, currently in a hypo, regulations state I cannot drive my vehicle until my sugars are higher. You can stand here and write me a ticket, or you can go to the shop just there and buy me a can of coke, and I will be fine to move my vehicle in 15 minutes” as you pass out a quid to him thinking him for his help and understanding. Then if he does write a ticket you will generally get it over turned if you fight it that he knew your situation and wanted you to break the law by moving the car.