Arent most of the goods people are stockpiling manufactured in the UK ?
No. Half of the food that the UK consumes comes from abroad, mostly either grown in the EU or imported via the EU, so any bottleneck at the border reduces the amount of food available.
UK food? 90% of vets in the UK food industry are from the EU, so if they bugger off, food production will be severely affected, which means less UK food in the shops. Actually about half of all workers employed in the food industry, from pickers to process staff to packers and drivers come from the EU. Likewise, if they go home, food does not get picked, manufactured or delivered. 'British' made food also requires ingredients. A frozen pizza uses cheese from Ireland and Italy, ham from Denmark, tomatoes and peppers from Spain and Portugal, mushrooms from Holland and so on. Packed in a carton made from Swedish paper. If any of these ingredients is missing, the whole process comes to a halt.
Medicines - 90% of medicines used in the UK (for humans and for animals) comes from the EU. Some of it can be stockpiled, but just like fruit and veg, many medicines are highly perishable, so any delays in their import can affect availability.
Other stuff - if there is a 10% reduction of port capacity, then there will be some shortages of food, goods and medicines etc. If there is a 25% reduction, things start to get more serious, with short-term shortages of even vital goods. But - HMRC and the freight companies fear that the reduction might be as high as 80%, in which case only one truck in five gets through. This would be close to catastrophic. Imagine that four out of five aisles in your supermarket would be empty. Goods would need to be prioritised, but if they prioritise medicines, then less food and no goods like paper for loo-rolls and packaging gets through. If they prioritise food, then less medicine gets through, and so on.