He left Hong Kong in 1886 to take control of Matheson & Co. in London responsible only to the firm's senior partner Sir Robert Jardine (1825–1905).

[2] Whilst in the colony, William also served as Consul-general for the Kingdom of Hawaii, for which he was made a Knight Commander of the Hawaiian Order of Kalakao (named in honour of Kalākaua, the country's last king).

[3] In 1888, Keswick and the chemist Herbert W. C. Tweddle bought the Negritos oil fields on the La Brea y Pariñas hacienda in Peru.

[4] After serving as High Sheriff of Surrey for 1897 he was elected Member of Parliament for Epsom at a by-election in 1899, and held the seat until his resignation on 8 March 1912 by the procedural device of accepting the post of Steward of the Manor of Northstead.

William died the day after this resignation at his home, Eastwood Park, Great Bookham, Surrey, on 9 March 1912 aged 77.