The UK used to have powerful AM stations which operated from ships outside of territorial waters, they're long gone except for Caroline which still exists (they even have their ship still) but as a legal station.

It's kind of challenging to do it on the land as a clandestine pirate since physics demands a large (as in hundreds of feet) aerial to radiate anything decent as well as usually a large set of radials for the aerial to work against. It's a more annoying version of the same problem amateur radio people have when using the 160 metre band. Of course it can be done, and people did it with much worse tech than what we have now, but it's not something to do at high power lightly. The voltage at the end of an AM/MW transmitter aerial is often pretty bracing for example.

Better call Christian Slater.