The music featured during the wedding ceremony is the movie composer Trevor Jones' atmospheric very short composition known as the Wedding Chant. Was this written specifically for the movie, or based on an actual chant?
I don't know much about any inspiration for Jones's orchestral melody, but the chanting is from the Christian Kyrie Eleison (an old piece that has Greek/Latin history; transliteration into 'Lord/Christ have mercy'). It's a liturgical chant which has been rendered with Celtic-like music (whether this is the case or not, I'm not sure, but Jones is known for his Celtic links).
There are other variants of the chant you can find on sites the likes of YouTube but the melody can be quite different (as opposed to the lyrics). Like I said, don't know if Jones created something original here.
Another thing, the official soundtrack only offers a snippet of the The Wedding, whereas a lengthier - and more vocal version - is used in the film (although the editing of the film has corrupted its flow when the scene switches between Arthur/Guinevere's wedding procession and the the chatter-boxing Merlin/Morgana). This version is hard to track down (for me, at least).
The Kyrie was part of the Latin Mass although the words are Greek. It is still a part of the Mass but in English. And, of course, there are many versions in chant.
I love the wedding scene and this version of the chant.