Treaty of London (1864)

The Treaty of London in 1864 resulted in Great Britain ceding the United States of the Ionian Islands to Greece. Britain had held an amical protectorate over the islands since the 1815 Treaty of Paris. 
The federated United States of the Ionian Islands included seven islands off the coasts of Epirus and the Peloponnese, that had remained in Venetian hands until 1797 and escaped Ottoman rule. Of the seven, six lay in the Ionian Sea, off the western coast of the Greek mainland. These six states were Corfù (Kerkyra), Ithaca, Paxò, Cephalonia, Zante (Zakynthos) and Santa Maura (Lefkas). Cerigo (Kythera) was also a state of the federation, although it is situated southeast of the Peloponnese.
Ever since Greece had become independent from the Ottoman Empire in 1832, the people of the Ionian islands had pressed for "enosis" with Greece. At a Cabinet meeting in 1862, British Foreign Secretary Palmerston decided to cede the islands to Greece. This policy was also favoured by Queen Victoria. The practical reasoning was that maintenance of ownership in the area was too expensive. Besides, the islands did not have great strategic importance; Britain would still maintain a strategic presence in the Mediterranean from the island of Malta. 
The decision to cede the islands was also influenced by the accession to the Greek throne of the Danish prince George, a committed Anglophile. Indeed, in a referendum in November 1862, the Greeks had elected Queen Victoria's second son, Prince Alfred, as their King, partly in the hope of receiving the Ionian Islands.
After long negotiations with Greece, the Treaty of London was signed by Greek delegate Charilaos Trikoupis on 29 March 1864. On 2 May 1864 the British departed and the Ionian Islands became three provinces of the Kingdom of Greece, although Britain retained use of the port on Corfu.
This can be seen as the first example of voluntary decolonization by Britain.