UK wanted to get rid of migrant workers because "they were taking people's jobs". So now they have no one to pick crops or drive transport vehicles. Food rotted in the fields, and products did not reach supermarket shelves due to lack of transport. UK citizens are not taking those jobs en masse because of low wages. As a result UK farmers have been reducing planted area (why waste money planting produce you cannot pick later). In addition, extra red tape in importation of produce due to Brexit means the UK cannot import food on time either.