When it comes right down to it, I am not really sure what the ultimate purpose is for this movie's existence. It does not seem to aspire to any higher purpose. It does show some rebellion against the government, but it is not something that is explored all that deeply. For that matter, once it reaches its conclusion, it does not seem like a whole lot has been changed. Fortunately, it doesn't matter. Pirate Radio exists solely to entertain its audience with some great music, a collection of colorful characters, and a tale of youthful rebellion of the past. Kids, pay attention, this could have been your parents 40 years ago.
Pirate Radio, released as The Boat That Rocked in the UK, is a fictional tale based on truth. It does not go so far as to claim "based on a true story" or "inspired by actual events" but I think you get the point. This is more about evoking the emotions and feel of the era without slavishly attempting to recreate a story of the time. It is a much better approach as it allows for more freedom to deliver interesting characters. For this decision, I applaud writer/director Richard Curtis, a man who has shown an ability to juggle large casts, making sure they all get their due and that you actually care about them and their place within the story.
Let's set the stage, shall we? In the 1950s and well into the '60s, BBC Radio played a mere 30 minutes of pop music. For some of that time it was illegal to play rock and roll. This has to be considered a travesty considering the wealth of rock and roll music at the time. Think about it — The Beatles, The Rolling Stones, The Who, Cream, The Kinks — the list goes on and it was not allowed on the radio. Certainly makes radio piracy seem like the right thing to do, right? To that end, rock and roll DJs took to the seas, broadcasting 24/7 from ships anchored in international waters off of coasts. There were a number of ships doing this and at any given point half of all the radios in the country were tuned to one of these ships, getting a nice dose of needed rock and roll.