Charity shops normally get their business rates bill cut by 80%. (The local authority has discretionary powers to increase the rate relief up to 100%). However, in order to qualify for the rates reduction, the majority of their trade must be in donated goods. If they sell too many new goods (including non-donated food products) the local authority can reduce their level of rate relief.
Otherwise there is absolutely nothing to prevent charity shops from selling food products. (Oxfam shops, for example, sell some 'Fairtrade' food items).
Chris