Cliffs of Dover is a piece of music without any vocals, played almost entirely on guitar.
In this version of the song, it opens with feedback and generally improvised plucking. The melody does not actually start until 0:26.
Since it is played by guitar and does not need to worry about the human vocal range like many melodies in popular music, Cliffs of Dover has a very wide range, going from very low to very high. Also, since guitars obviously don't need to breathe, the phrases are quite long.
While there are some very obviously jarring and disjunct notes, the song as a whole has a very smooth contour, sliding easily up and down in pitch.
While singing something based on this melody would probably be fairly easy, due in part to the generally smooth contour, trying to sing every note in the actual melody of this song would probably be a nightmare. The notes come out very rapidly and in very long phrases, and they also change pitch very rapidly. This melody was not meant to be sung, but rather it was meant to be played in the guitar or something similar.
Contrary to Will I, Cliffs of Dover is all about embellishment and showing off. The rapid speed of the song and changing notes indicates that well enough, but many of the notes are also accented with vibrato.