From the Brooks Web site:
The new Leather Barbican is manufactured in England using the same hardwearing leather used for Brooks Saddles. The Barbican was first developed and is still available in cotton with leather details. Both versions of the bag feature an adjustable shoulder strap that can be easily secured around the waist. The strap can be worn loose to carry the bag over the shoulder while walking. Then, for increased comfort while riding, the two ends of the strap can be pulled together around the waist and fixed as a ‘Sam Brown’ belt.
Who Sam Brown is, I have no idea. But I know "hardwearing leather" when I see it.