From the YouTube notes:
The communion hymn from Midnight Mass 2011, broadcast live from St George's Cathedral Souhwark. The hymn is sung by the Choir of St George's Cathedral, the Cathedral Girls Choir and the congregation. The verse harmonisation is by Sir David Willcocks, the last verse and descant by David Briggs. The Choir is director by Nick Gale with Norman Harper at the organ.
St George's Cathedral, Southwark is the cathedral of the Roman Catholic Archdiocese of Southwark, south London and is the seat of the Archbishop of Southwark.