From the Brooks website:

"Our Roll Up Panniers are inspired by the Cyclist’s Hold-alls, first
featured in the 1890’s catalogue. As the originals, the new panniers can
be rolled up or easily released when needed.
This pair of commuter bags for traditional transport bicycles is
entirely made of water resistant cotton with leather finishing. On the
top of the bag attachment to the bicycle rack these panniers feature a
crossed elastic to fasten additional objects to carry during urban
transit. Last, but not least, the Roll Up Panniers are “self closing”,
as they feature magnets on the flaps.

Materials: water resistant fabric with leather finishing

The Brick Lane roll up panniers attach to most racks with a width between 10 and 15 cm."