Who Would Enslave Britons in the Eighteenth Century?

 What threatened Britain in the 18th century that Britons would not be slaves, as in the refrain of Rule, Britannia. The poetry was written by James Thomson, a Scot for whom there's not much other information easily available. 

The year of the poem is 1740 and it turns out the poem was written after the British took Porto Bello in the War of Jenkin's Ear.  Fears of the Jacobite Rebellion of 1745 might have been a factor, since France and Spain were supporting the Stuart pretender to the throne by 1743. So it's wartime when he's writing, the Brits have a victory, and the poem is both boastful and anxious.

The "slavery" in the poem is rhetorical, not descriptive of anything like chattel slavery.