Since I first got my Brompton I have been using the touring bag for every trip, whether I needed all that space or not. I tucked my repair tools in one of the side pockets and used the bottle pocket to carry a water bottle. It was my only choice if I wanted to have my tools and pump with me and carry some water. It's about the biggest solution to an everyday need.
I considered other options for the luggage block, and right now, I just can't afford the smaller bags that are available through Brompton. I looked at saddle bags, and some are very slick. The Carradice Zipped Roll and Cape Roll are very nice, but I just wasn't willing yet to spend all that money on something I couldn't see and touch. (Funny... I didn't feel that way about my Brompton!) There's no dealer nearby that sells them. The Brooks saddle bags are totally sexy, but small and very expensive. When I was in Boston, I did see a Banjo Brothers saddle bag, but I wasn't sold on it. I wondered if it was too big.
In the end, I took the cheap way out to figure out what I really wanted. For less than $10 I ordered the Avenir Metro 2.0 Seat Bag. It's advertised as 140 cubic-inches but is in reality about 100 cubic-inches. That's about 1.6 L. It's made of a nylon cloth and is not waterproof or really even water resistent. One zipper opens the bag and two pressure buckles attach it to the Brooks saddle loops. It does have a place where I can move my rear flasher from under my seat to a loop on the back of the bag. There is a sheet of plastic that slips into small pockets on either side of the zipper to keep the bag in a roll shape.
It is certainly nothing fancy, and in it fits my tools and pump, some snacks, my wallet, my keys and a rain shell if it looks threatening. For the price, I'm very pleased.
I'm sure that in the future I would like to get something a little bigger. Perhaps I can buy a S luggage frame and fix up something that fits that costs less than the $250 for another Brompton bag.