I am reading Kissinger's book "Diplomacy" and the first few sections of the book deal with the system of balance of power in Europe. One of the main things Kissinger keeps repeating is that Great Britain almost always acted in the interests of the weaker side in any conflict in Europe in order to preserve the balance of power. My question is then, why didn't Great Britain join French in the Franco-Prussian war in which it was clear that Prussian victory would alter the balance of power significantly?
First of all, France was seen as the stronger side in this conflict. Her army wasn't only rated as a most formidable force, it was even unclear if the southern german states would join the war and how strong Prussia's army would be in a war against an enemy like the French. The main concern was to keep the conflict as localized as possible, especially with Russia keeping benevolent neutrality towards Prussia.
Then, in fact, I have to tell you, that Britain was actually in favour of Prussia. At least before and during the war. Not only did Britain and France have a long history of rivalry and war while Britain and Prussia had always been allied or, when not, had never fought directly against each other, both were also protestant while France was catholic. And a stronger Prussia or even a unified Lesser Germany as a buffer state between France and Russia and also Britain and Russia worked for the British just as well or even better than the previous status quo. Inacceptable would have been a Greater Germany, but that wasn't expected as a possibility at this point.
Besides of that there was actually the fear, that France would conquer significant amounts of german territory and thus alter the balance of power, let alone the fear that had been built up since 1866 (Luxembourg Crisis, Belgian Railway Crisis of 1869, Napoleon III demanding prussian neutrality towards a french annexation of Belgium in exchange for french neutrality towards the prussian restructuring of Germany), that France would actually garner control over Belgium, decreasing the distance between France and Britain and that way increasing the threat of invasion of british soil.
Also the british public opinion felt, that France was acting irrational and unnecessarily hostile towards Prussia during the Crisis over the Spanish Throne, Leopold of Hohenzollern-Sigmaringen and the Ems Telegram and that France declared war over virtually nothing.
Sources:
Hildebrand, Klaus No Intervention. Die Pax Britannica und Preußen 1865/66 - !869/70.
Schmidt, Jan Hendrik Der unterschätzte Krieg: Europa und der deutsch-französische Krieg von 1870/1871