The Treaty of London, 1878
Following the conclusion of the Russo-Turkish War [1], the two sides agreed to a peace settlement, eventually settling on the terms of the Treaty of Adrianople [2]. This treaty, however alarmed the other Great Powers, particularly the Austrians, who viewed any major Russian expansion ino the Balkans as a strong upset of the status quo.
As a result the Great Powers, and the parties of the Balkans agreed to a second treaty negotiation to be held in London: the result would be of great surprise to the Russians, who found an unexpected ally with William Gladstone, who agreed to the formation of the Bulgarian state, though he vetoed any annexation of Ottoman Macedonia to the new state. [3] Nevertheless, Britain and Germany (despite the ruminations of Austria) agreed to the original treaty's articles transforming Bosnia into a autonomous part of the Ottoman Empire, the recognition of independence for Serbia, Montenegro and Romania and the granting of neutral shipping rights to the Straights of the Bosphorus and the Dardanelles.
While the treaty would improve relations between Britain and Russia [4], the reaction from the Conservative opposition and press, was near hysterical, with Gladstone accused of selling out the Ottomans to Russian perfidy. Nevertheless, Gladstone ensured that the new Bulgarian state would not be fully Russianised by convincing the other Great Powers to back the British supported claimant for the throne, twenty year old Prince Valdemar of Denmark [5], who was related to both royal houses through his relatives marriages.
The Balkans was a very different place from 1876...
[1] Pretty much follows the same course as OTL, though the Russians don't do quite so well in the Caucasus, with the Ottomans maintaining hold of their Armenian provinces (as the Russians were unable to conquer them.)
[2] Pretty much identical to OTL Treaty of San Stefano.
[3] William Gladstone, when in opposition during the late 1870s fully opposed British actions in regards to the Balkans and was a supporter of Bulgarian nationhood: hence Britain supporting an independent (though not the Greater Bulgaria envisioned by the Russians) Bulgaria.
[4] So much so that the Anglo-Afghan War of 1878-80 is butterfiled away: though the two nations are still wary of each other, and Britain finds itself closer to the liberal Germany.
[5] Younger brother of OTL George I of Greece, he ascends the throne in late 1878 as Alexander I of Bulgaria.