England (and its succeeding states) were at war with France for close to 1000 years. First it was over land, then they competed over religion, then they competed over colonies.
For much of that time, England was allied with various German states.
All of a sudden, in World War 1 the UK found itself an ally with France and at war with Germany.
What caused this change?
British policy on the continent has traditionally been to maintain the balance of power (this is also really a general European thing). This amounted to shifting alliances all over the continent. Though France and Britain are "traditional" enemies (as neighbours were wont to be in Europe), they certainly hadn't been at war for anywhere near "close to 1000 years".
In fact Britain allying with France isn't that unusual. England supported the League of Cognac, an alliance of France and some Italian states against Germany and was close to joining. Britain and France were on the same side against Spain several times, including the War of Portuguese Succession, the Carlist War, and the War of the Quadruple Alliance. In the 17th century there was an abortive alliance under the Protectorate. In the 18th century another alliance was relatively successful against Russians. And in the 19th century, Anglo-French troops fought together against Russia in the Crimean War.
A key change after the mid-19th century is the emergence of Germany as the continent's dominant military, economic and industrial power. This is particularly demonstrated by her victory over France in 1870. It was pretty natural for Britain to seek counterbalancing allies.
I agree with much of Semaphore's answer, which shows that actually Britain and France were not in a state of perpetual war.
But I think your question really relates to "What changed?" so I'll try to answer that.
Firstly, the end of the Napoleonic era. The Battle of Waterloo and following months were the end of the Napoleonic wars, and the end of the "Big" conflict between Britain and France, that of Empire and Conquest.
These wars had been, for over 100 years, about trying to take the "best" territories, colonies, etc. from each other (And in this I include Britain, Spain, Portugal, France and The Netherlands, plus Russia, Sweden, Austria to a lesser extent). During the Napoleonic wars, Britain had fairly clearly gained the upper hand and had taken most of the desirable colonies. With the end of the war, France wasn't really in a position to try to take them back, nor did it (temporarily) have the willpower for further conflict.
For a short time afterward, Britain and France both had their hands full just trying to keep hold of their current possessions, France particularly since it was also trying to rebuild at home, but Britain had her own problems... rapid expansion is easy, keeping hold of those possessions isn't. The fact that Austria, Russia, Prussia (soon to be Germany), Spain, The Netherlands and the United Kingdom were all "keeping an eye on" France also meant that France, by necessity, had to maintain a fairly cautious foreign policy.
Fast Forward around 50 years, though, and although there were a couple of "Ooh, this is getting a bit tense" moments as France started to flex her muscles again, France and Britain were both fairly happy with their lot and far more concerned with trade than expansion. Both had recognised that they had little to gain from war, and shared many of the same interests. However, both were starting to become concerned about Russia and China, particularly, and the balance of power in Europe.
Next, we have some "Alliances of convenience" - The Second Opium War and perhaps more crucially the Crimean War, where France and Britain's interests aligned closely. It was in both of their interest to deal with these "situations", so they did. These joint-expeditions paved the way for closer ties.
Another 20 years, and we can see that Britain and France are sharing even more concerns: America is first embroiled in civil war (with a major impact on export to Europe, particularly the food from the Union France needs, and the Cotton from the Confederacy to feed Britain's textile industry). And after the Civil War America starts growing in confidence and starting to look outward, beyond her own borders.
Then Germany, already acknowledged (as Prussia) as having a strong military tradition ("Where some states have an army, the Prussian Army has a state!" - Voltaire), starts to expand. German Unification, and its closer ties with Austria, start to throw European politics out of balance. France and Britain had been fairly effectively keeping each other in check for some time, and actually hadn't been heavily involved ON the continent for nearly a century... but suddenly Germany was starting to look threatening. Militarily to France, exerting influence over regions that had "looked to" France for guidance, and obviously by sheer proximity, and threatening to Britain's burgeoning economy. Both were (rightly) fearful that Germany's increasing strength would lead to conflict, and their alliance was borne out of that.
And, essentially, that is how France and Britain became allies: through realizing that they had little to gain from war, were fairly well balanced, were threatened by the same (potential) enemies, and had mutual interests. There's a lot more to it than this, however, if you're interested in really getting into it.
In german history lessons (as I remember them) the main reasons are listed like this:
I'll allow myself a personal side note: I think Bismarck was right in choosing allies and pacifying them by not taking part in the race for colonies. In contrast, the German King Wilhelm II was an idiot for not seeing the intricate plans of Bismarck and the necessity of allies and peaceful foreign policy. It was under King Willi (as I like to call him rather unflatteringly) that Bismarck and his policies fell out of favor.
The Fashoda Incident was the culmination of Britain's North-South expansion through Africa colliding with France's East-West expansion. As Germany was increasing its dominance in Europe, England had to grab a major ally in Europe and their choices were either Germany or France.
The Boer wars largely arose from Britain's mismanagement in Africa and since France had a much greater African presence than Germany, that probably prompted them to side with France.
After the Fashoda Incident, Britain and France set out on a path towards much closer friendship.
Britain tended to ally with the second strongest country on the Continent against the FIRST. This was often for balance of power reasons. For many of those "close to 1000 years," France (with the best climate and largest population in western Europe) was the strongest country, and the greatest threat to Britain, and Britain would ally with others (e.g. small German states) against her.
But after the Franco-Prussian War, the defeat of France, and the unification of Germany, Germany became the strongest Continental power (and a very aggressive one), France second, and Britain allied with the second (France) against the first (Germany).
Under the circumstances, the change of alliances was not surprising. As Lord Palmerston would say, "We have no eternal alliances and no perpetual enemies. Only our interests are eternal and perpetual."
Enmity between Spain and Great Britain started when new royal family started in Spain. Until the Catholic kings Ferdinand and Isabella decided to join to the Austrian family marrying theirs sons Spain was only interested to fight against Islam. That provoked the union of families and also with the Austrian conservative ideology.
For this Spanish Austrian family the principal European enemy was France apart from the Muslim world. As long as the UK was pro-Austrian and against France, the alliance and friendship was total. However, with the arrival of the breakdown the Christian religion that alliance was broken.
Between XVI-XVIII century Europe was divided between two powers: The Austrian-German Alliance and the French Empire. UK joined France until the schism in the German-Austrian Empire that weakened the Austrian Conservative family. In XVIII, Austrian Family lost Spain. For the European great powers, Austrian power was destroyed. Becoming French and UK as the only big powers of Europe.
In early XIX.century, with Napoleon defeated, UK was the only giant of Europe while the rest of Europe countries were extremely weak. All countries of europe, decided to start the industrial revolution but with two visions: Protectionism (the resurrected Germany) or Liberalism(UK). France decided for UK and liberalism, Austria with Germany and protectionism.
Spain was divided, weak and in civil war until the Second half of XIX, joining with Liberalism of UK signed an agreement in 1906 with Alfonso XIII. That provoked the economic growth of Spain. Train, textile industry, electricity, shipbuilding, car industry, aircraft industry etc... until September 1923 with the withdrawal of British investments. With the Primo de Rivera dictatorship between 1923-1929, Spain chose the protectionism but decided to maintain relationship with allies necessary to win Rif War. Spain's pro-German or pro-UK attitude varied depending on the political situation.
Britain had not been involved in the outbreak of WWI, as Britain had no alliances that could force them to enter the war. If Britain actually entered the war was not in favor of France but in favor of themselves.
The main reason of Britain to enter the war was to protect their vast global empire from being affected by the war. Additionally, Britain saw the potential defeat of France as a very negative outcome for their maritime control, if Germany would eventually control the havens of the North Sea and the English Channel.
