The usual running joke is that the English and French were always at war throughout history, but today the thought is unthinkable really. They went from staunch adversaries at Waterloo to Triple Entente allies in roughly 100 years. So, what changed? Was is the scale of the World Wars that forced them to be allies against a greater threat? Was it the end of the Napoleonic era that allowed for closer relationships? How did France and the United Kingdom (mainly England) become friends?
Many would say they never were and are not 'friends'!
Prior to the First World War, the British and French were rivals and enemies, and each side's military developed strategies for attacking and defending against the other. This was occurring remarkably close to the First World War.
It was the rise of Germany that changed things. The British monarchy was very 'German' before and during Napoleon's time, a legacy of the Seven Years' War when the British had bankrolled armies in the Germanic states to fight the French on their behalf (while they got on with taking the colonies). Famously, the kings of Europe in 1914 were all related in part because of this. While France was the major European power, this strategy suited Britain well, ensuring the European nations looked suspiciously at each other. A single hegemon would be a threat to Britain's global empire.
The 'second industrial revolution', that built on new technologies and sciences rather than factories and cotton, was a boon for the newly-unified Germany. She went from being a semi-pastoral economy to being an industrial leader in mere decades. But, Germany had risen too late to gain colonies, resources and so on and felt 'shut out' of the trappings of power Germans felt they deserved.
Among the results of this, the Germans instituted policies such as building a modern and well-armed navy, a threat Britain could not ignore. This led to the British seeking allies, signing the Entente with France and Russia. This was not really a proper alliance, more an understanding with plenty of escape clauses. Despite it, British-French joint planning for the event of war was minimal.
The First World War was the great 'unifying' event. Britain, with their tiny army, started as the junior partner but as the French smashed their army and Britain mobilised her strength, this shifted. British-French co-operation was far from efficient, but as the war progressed they became more dependent on one another. The big German assault of March 1918 (100th anniversary of these events right now) created a crisis, as the British prepared to pull back to the English Channel and their escape/supply routes, and the French prepared to fall back in the opposite direction to cover Paris. This led to the appointment of French General Foch as overall commander of the British-French forces.
In the Second World War the British and French were far from friendly. The Germans neatly pulled off in 1940 what they failed to do in 1918, and France effectively surrendered. They felt betrayed by the British, especially when the Royal Navy attacked a large part of the (surrendered) French fleet, at anchor, and sunk it with heavy loss of life. De Gaulle's Free French were unimportant until 1944, just a small force of dubious legitimacy, and only in Britain due to a lack of options. Some French preferred to go to the Soviet Union to fight Germany. The attitude of the Free French and the British was always one of resentment.
Postwar, the British and French retain the legacy of this history. There are few compelling reasons for them to hate or fear one another, let alone make military preparations, but rivalry rather than friendliness is the key. France resisted joining NATO, and developed its own nuclear arms capability. The desire of European unity has led to many political leaders to seek a break from past rivalries, but Brexit shows this is a long way from permeating the people and cultures of the nations. Brexit negotiations will reveal just how 'friendly' the two states are, and i expect them to be harsh.