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.