I have to admit that I had not heard the processional song from the Royal Wedding of Will and Kate, but it was so regal and beautiful For anyone else whom may not have heard it is the choral anthem " I Was Glad" composed by Sir Charles Hubert Hasting Parry. It was originally composed for the coronation of William's 3 - time great grandfather, Edward VII at Westminster Abbey back in 1902. Most of the content of the psalm is a prayer for the peace and prosperity of Jerusalem.
I am thinking that it could be scaled down to something simpler - if you don't have a full orchestra, and choir booked for you wedding. The melody and harmonies are amazing.