What I am trying to get at is: what did the British people consider the greatest threat to their own self-determination? As 'Rule Britannia' famously says: "Britons never, ever shall be slaves."
So what was the closest that Britain got (or the closest Britons thought they were) to losing their sovereignty in actuality? Was it the threat of the Spanish Armada? Napoleon? Hitler's Battle of Britain? I know that this question is incredibly broad and may be difficult to answer, but I am interested to hear the opinions of this community. Any information would be appreciated!
The interesting question is how much Thomson (who wrote the Alfred masque from which Rule Britania comes from in 1740) was channeling from his earlier work on Carthaginian princess Sophonisba (The Tragedy of Sophonisba) when he wrote that line. Certainly in 1740 Britain did not rule the waves (the Dutch would still have had a few things to say).
You will need to define your periods too. Britain as a political entity covers 1707 to 1801 when Ireland was added (so United Kingdom of Great Britain and Ireland. Prior to 1707 you're dealing with the separate nations of the island(s) of Britain.