The front panniers are the best way to carry things on the Brompton. There are four possibilities:

Cloth Pannier: Briefcase-shaped, with one main pocket, a flat back pouch, and two back pockets. 18l capacity.

Touring Pannier: Larger (28l), rounder, with roll-top closures on the main pocket and back pockets, and external mesh pockets.

Folding Basket: Open-topped, with enough capacity for two full shopping bags.

S-Bag: The only bag that will work on the S-Type bikes because of the lower bars. This is courier-bag-like, with a fold-over flap, and loads of internal and external pockets.