The Brooks Barbican shoulder bag will make you the envy of your office. Perfect of all on- and off-bike excursions.

From the Brooks website:

"In 1910 John Boultbee and his son Wilfred Mason Brooks filed two
patents related to improvements in satchels, knapsacks and the like.
Both these patents guaranteed more equal distribution of the load over
the straps and more stability of the bag whilst cycling. With this in
mind we developed our new BARBICAN Shoulder Bag, featuring a shoulder
belt which can be easily secured around the waist during the ride.

The belt is the innovative part of this bag. Once adjusted to the
desired length, depending on the height and waist of the cyclist, the
belt can be kept loose to carry the bag over or across the shoulder
while walking. For increased stability during the ride, the belt can be
easily fastened around the waist simply pulling the two ends of the
belt and locking them at the center with the “Sam Brown” fixing."

Water Resistant Cotton Fabric and Vegetable Tanned Leather from Europe