Battle of Britain
Although not an Army battle it was one of the defining moments in the country's history.
The British Expeditionary Force had been pushed back to mainland Britain, there was no sign of the USA joining the war and the German Forces were planning invasion. It was just the Fighter Squadrons of the RAF that stood between the Luftwaffe getting domination of the skies over Southern England, something needed for a successful invasion to be possible.
The RAF inflicted such high losses on the Luftwaffe that they shifted their main efforts from daytime to night bombing and as such never got control of the sky and all invasion plans were effectively shelved. Hitler decided to concentrate on the East and Britain was given respite until the US joined the war and plans could be made to invade Europe.
Controversy remains as to whether the RAF really was the last line of defence between Britain and that jumped up corporal with the single bollock and his army of storm troopers. The Luftwaffe came so close to defeating the RAF (they decide to start pulverising the crap out of London right before the RAF was about to pack it in and retreat) and had they done, the Germans could have invaded - or could they? After all, saying they could have invaded immediately blithely disreagrds all the home defence squadrons of battleships that the senior service had parked on the British coast.
If the Germans had invaded, they were using flat bottomed barges which would have made the crossing at a safe speed (to stop the, let's face it, crappy British weather form sinking them) and this would have taken 22 hours. 22 hours! Poor b*stards in the boats! All it would have taken to defeat this flotilla would have been a single destroyer, not even a proper battleship, chugging up and down the invasion route at night, when the Luftwaffe showed as much ineptitude at bombing as their RAF counter parts (the German navy was too small to pose a threat to the numbers of home defence ships). The destroyers didn't even need to crash into the barges - sailing past would have been enough to capsize them!
However, at the end of the day, it never came to that, as the RAF did hold out until the Luftwaffe exhibited a brilliant lack of attention span and started try to smash the British population.