@thatbags
They can, if they're prepared to pay the import tariffs. Have you ever tried to send a Christmas present to somewhere like Canada? Sometimes the import tariff can be more than the value of the gift.
If you're a small, niche business, whose customers are prepared to pay the tariffs, there's not really a problem. However, if you're a large manufacturer, whose business is critical to a country's economy, it's not a very sensible way to go about things.
Car manufacturer is a good example. The UK industry is recovering and is one of our biggest exports, but only because we are able to compete against EU manufacturers. If the EU slapped a tariff on British made cars, it wouldn't be long before Nissan and other manufacturers decided it was too expensive and too much hassle. At the moment a car manufactured in the UK is made up of components from all over the EU. Can you imagine what it would be like trying to work out which part was manufactured in Spain, Germany, Poland, the UK or whatever? Nissan can also switch production from Spain to the UK (and vice versa) at the touch of a button if one factory is working to capacity. Customers are guaranteed the same price wherever the car is produced, but it would be more complicated and less efficient for Nissan if didn't have the capacity to switch.
Tariffs can be used to control imports and exports, which ultimately have an effect on jobs, etc. The UK blocked an attempt by the EU to impose tariffs on Chinese steel, because it wanted to continue importing cheap steel, but at the cost of thousands of UK jobs.
Surely you remember all the stuff from your history lessons about the Corn Laws in the 1830s. ;-)