Westminster Cathedral Choir singing the Advent Prose (Rorate Caeli - Drop down ye Heavens from Above, and let the skies pour down righteousness). Westminster Cathedral really do have a great choir, especially when it comes to Gregorian chant.
"O Come, O Come, Emmanuel" by the Choir of Clare College, Cambridge (the church is actually the cathedral at Bury St Edmunds, though).
"Lo He Comes With Clouds Desending" - Lichfield Cathedral Choir (taking it rather slow, but it's still a cracking hymn).
"Wachet Auf" - Nikolaus Harnoncourt and the Concertus Musicus Wien (nb: this is 28 mins long...)