Our food prices are already amongst the cheapest in Europe, imports are likely to go up in price because of costs involved in the increased red tape and delays in the supply chain.
https://www.mirror.co.uk/news/politics/brexit-deal-affect-food-prices-23217289
Reading the quoted article more closely, it seems that prices may rise 4.7% on unbranded products. An example of unbranded products reaching the UK would be products from China being sent to the UK via Albania. Food imports may suffer due to delays, but the UK could switch suppliers if necessary.
"the UK
may still experience a hike in food prices due to 'non-tariff barrier costs' at the border with the EU"
"Even with a trade deal in place, our access to the EU market will face significant barriers after December 31 - and food prices
may rise."
"businesses will still need to carry out customs declarations on UK-EU trade for the first time. These 'non-tariff barrier costs' are expected to rise by 4% to 8%, according to farmers' bodies.
"The London School of Economics estimates that with a variety of other costs adding up, there’ll be a 4.7% price rise on
unbranded products from the EU. It's
possible that this price hike will be passed on to the consumer."