Exactly the bag I needed
After a few weeks of test, I am very happy with the bag. It is well made, waterproof and is large enough for my needs.
Goes quite well on my Brompton with M handlebar. Works also well as a normal backpack once the frame is removed.
Only one tiny issue: the bag being a bit tall, you can't really put it on/off the bike when in parking position, since the handlebar will be a bit twisted and be on the way of the bag. But that's a minor thing.
Otherwise, it's a very good bag, exactly what I needed for my daily commute. I recommend it.