Exactly what the key moments in history in the UK are is debatable, but this list from Professor Robert Tombs of Cambridge is interesting. Professor Tombs is, incidentally, a professor of French history, not British. Here is his list:
500 Anglo-Saxon conquest
900 The Danelaw
1066 The Norman conquest
1170 Battle between Henry II and Church
1215 Battle between King John and barons
1258 Consultation of barons and townsmen
1314 Scottish and Welsh wars
1381 Peasants’ revolt
1415 Agincourt and 100 Years’ War
1485 Defeat of Yorkists
1535 Execution of Thomas More
1553 Catholic Restoration
1588 Defeat of Spanish Armada
1611 King James Bible
1649 Execution of Charles I and Cromwellian conquest of Ireland
1689 Glorious Revolution and Battle of the Boyne (1690)
1707 Union of England and Scotland
1721 Beginning of Walpole’s administration
1763 Treaty of Paris, securing Canada for Crown
1769 Spinning Mule invented: key moment in Industrial Revolution
1776 Loss of 13 Colonies
1805 Battle of Trafalgar
1825 Opening of first railway
1832 Reform Bill and 1833 Abolition of Slavery
1838 Chartists and 1846 Repeal of COrn Laws
1886 Irish Home Rule Bill
1904 Entente Cordiale
1919 Treaty of Versailles
1931 Statute of Westminster
1940 Churchill as Prime Minister
1945 Labour landslide
1947 Indian independence
1960 Wind of Change
1969 British troops deployed in Ulster
1973 UK joins EEC
1984-5 Miners’ strike
1997 Tony Blair elected to power
Nothing on the ancient Romans or the ancient Britons other than Boadicea, who was a loser and less important than the two thousand years that preceded her. I'm not at all sure I'd have included Robert Walpole or the Entente Cordiale, which was less important than WW1. There should have been a place, too, for the Battle of Britain. We were very fortunate that Hitler started to make mistakes just when he could probably have finished us off.