The Brompton Folding Basket is many Bromptonauts' initiation into how useful their Brommie can be for transporting cargo - be it an existing bag, backpack or even loose shopping.
The Basket Bag is an evolution, which is full of win. A curved frame provides a better shaped bag. They've also added a large internal pocket so you can have ready access to key items. Either side, there's cinch straps to stop the bag rocking in/out when riding. There's a large reflective stripe to keep you visible at night. Finally, there's handle loops which will automatically close the bag when you lift it, or grab one loop and you have a handy shoulder strap.
The Brompton Basket Bag - if you're only going to get one item of Brompton luggage, it's a good choice!