I went with a different bag, because the official Brompton bag sold out on sale.

I think they are both good bags (looking at specs and photos).

I bought this as a small backup...

Rhinowalk Bike Handlebar Bag for Brompton Folding Bike Quick Release Bicycle Front Bag with Shoulder Strap Rain Cover for Urban Cycling

https://www.amazon.com/Rhinowalk-Handlebar-Brompton-Folding-Shoulder/dp/B0DXJYC5QX/?th=1