They are allowed to put unwrapped food into small bags.
"Small retailers don’t have to charge but can do so on a voluntary basis. You can avoid paying the charge either by reusing single use plastic carrier bags, or by using multi-use bags for life.
You also won’t pay a five pence charge if you’re using a paper bag, if you’re in transit, or if your bag only contains certain items, such as unwrapped food, raw meat and fish, prescription medicines, uncovered blades, seeds, bulbs and flowers, or live fish."
From www.gov.uk/government/publications/single-use-plastic-carrier-bags-why-were-introducing-the-charge/single-use-plastic-carrier-bags-why-were-introducing-the-charge
"the average household already has 40 plastic bags around the home" so that is an average of 40 bags that they can put into the car, the handbag and so on. That will cushion the transition for a while.
Live fish!
Imagine carrying them loose in your handbag . . .