The readings for the Baptism of Christ are:
The reading from Genesis is about the first day of creation, with a lovely focus on water and light.
The psalm is a vivid depiction of God’s glory in nature. This would make any short Gloria text appropriate; for example, Eleanor Daley’s Gloria in Excelsis Deo or, for something a bit calmer, Gloria by Sakari Dixon.
The reading from Acts is about the baptism in the name of Jesus, by which Christians receive the Holy Spirit.
The Gospel reading, from Mark, sets the scene of John the Baptizer calling people to repentance and baptizing them, and Jesus being baptized by John.
Both Acts and Mark mention the Holy Spirit, so music related to the Holy Spirit would also be appropriate.
Technically I think we’re not supposed to move Epiphany to the nearest Sunday, but I’m sure a number of churches do. In that case the readings would be:
Isaiah seems to be saying to prepare for a better time, a gathering-in, being recognised by nations — and there’s the gold and frankincense, foreshadowing, if you like, the gifts of the Magi.
Psalm 72 is a prayer for support for the king, and continues with the theme of expensive gifts being brought from easterly places.
In Ephesians, Paul tells the receivers of his letter that the inheritance with Christ is not just for the people of Israel but for them, gentiles, too.
In Matthew’s telling of the visit of the Magi there’s a strong focus on how scared Herod is of the promised Messiah, but the other essential details are there too: the star, and the gifts.
As an anthem, I walked in darkness by Kathryn Rose would work — though it seems to be done just as often during Advent and Christmas, when choirs haven’t just been on holiday for a week or two.
For either of these services, Christ be our light by Bernadette Farrell could be appropriate, especially if there’s a particular focus on light.