I have an S6L..I have an S type bag..Before I got the S bag I had an old Carradice bag made in the 1990s for a Brompton.This was quite a bit bigger and carried a fair amountof stuff..The S bag is good though .You can get a lot of stuff in the zipped part of the bag on the top flap..
I have an old Carradice Nelson long flap saddlebag that I use more than my S bag..I have an old karrimor quick release on the bag which means I can take it off in seconds..You can use an SQR Quick release from Carradice to fit a bag to ..or you can get a quick release Bagman from Carradice..You can also buy a bag and quick release from Klick Fix..to fit on your Brompton..
But be warned a Carradice bag on the back of a Brompton is not a good idea..OK with 5 kilos or so in the bag..But with 10 kgs or so the bike will tip up backwards when the rear wheel is folded underneath ..And of more concern is the fact that it puts a stress on the seatpost and the frame...