This song is Sir Hubert Parry's music to a short poem by William Blake. The poem is based on a myth that Jesus would visit England and create a Heaven on Earth. The Song was originally called "And Did Those Feet in Ancient Time" but was changed to "Jerusalem" after its composition for many arrangements and performances. This is the most popular work by Parry, and Prince Charles is a fan of the composer's work and has promoted it in a BBC Television Programme called "The Prince and The Composer" broadcast in June 2011. The Song has become a Patriotic Anthem sung at the Last Night of the Proms (usually in an orchestral arrangement by Edward Elgar) and sung at other events such as Rugby matches as an unofficial National Anthem of England. Because of the poem's religious sentiments, this Song is frequently used as a Hymn in Anglican and other Christian Churches. The title of the film "Chariots of Fire" derives from words in the poem, and the song was sung on the film's soundtrack.
Here is a video of Parry's Jerusalem for piano:
The image below is the first page of the sheet music, and here you can download the full sheet music for Parry's Jerusalem for piano in PDF format.