Buying locally vs abroad
The currency is not as relevant as so many other more important factors. 

Buying a better ship is a very important factor. If a better ship is chosen and it is Korean, then the buyer (and seller) will benefit from their comparative advantages. I.e. instead of making British workers produce inferior ship, they can produce something they are better at instead, sell that to Koreans, and buy a better ship from them.

Another factor is the fairness of trade. Is the trade fair? Are there any tariffs, subsidies, dumping, theft, currency manipulation, corruption, extortion, forced labor, etc. etc. going on? Maybe not in South Korea and UK, but generally international trade is never perfect.

National security. Are you buying a military ship? A tanker? Is the deal big enough to be abused for political purposes?

There are many more of course.

Currency element is just that one item the list that I highlighted.

That's why big deals like ships, which are more like into 10 billion pounds, usually require a lengthy public review process to go over all of these issues.

On the other hand, if we are talking about 10 pound soda bottles, then currency is about irrelevant for large to medium businesses that can fix exchange rates using Forex futures or swap contracts at virtually no cost. Different currencies is a problem for smaller businesses because accessing heavily regulated financial markets is pretty tough for them and they usually have to bear exchange rate risks and pay hefty commissions to the local banks.