Mike just like most countries in the world we don’t make everything or produce everything so trade between countries would continue in the same way as it does now.
After we leave the transition period, with or without a deal, we will be in a position we have not been in for many years. If we buy British it reduces the pressure on trading with other countries. We do of course want trade, and lots of it but the next few years will be tricky and could be very difficult. So the more we can do to help ourselves the better.
The next time you are in the supermarket have a loôk at the butter your buying or the mince and see if they are British, if not look beside them and see if there are made in Britain alternatives we need to look after ourselves in these difficult times.