18th century Britain was quiet? We started the century with the War of the Spanish Succession, also had the War of Jenkins Ear, the War of the Austrian Succession, the American War of Independence, 3 Jacobite rebellions (including some of the last battles on British soil), the Battle of the Nile, I think there was some trouble in India, and also the French Revolution impacted on Britain - so not really a quiet century at all.
I don't think that there has ever been a time when there has been no war in the world - active or cold war - if it seems as there has been a time, it probably just means historical records have been lost!