The last three Secrets to collect are in Westminster, dotted around Buckingham.
Secret #01 - Buckingham
This one is in the northeast corner of St. James's Park.
Enter the park from the northeast.
Head over to the hut on the grass in front of you.
The music box is at the front of the hut on a bench.
Secret #04 - Buckingham
The second music box is at the southwest corner of St. James's Park.
There's a willow tree in this corner of the park that you can see from the Buckingham Palace viewpoint. Make your way over to it.
The music box is on a crate beneath the tree.
Secret #30 - Buckingham
The last Secret is at the southeast corner of Green Park, to the north of Buckingham Palace.
Enter the gate at this end of the park, from the street that runs around the outside of Buckingham Palace.
The music box is on a tree stump in front of the gate.