I am looking for a way to keep a few essential items permanently on my Brompton. These items are: small U lock, front headlamp, second rear flasher, tube, tire levers, axle nut spanner, patch kit, multi tool, chain tool, bag cover. I am currently using a Radical designs saddle bag which is ok, but is hard to dig out the items in the bag due to the tight fit with the bag-cover folded inside the saddlebag. I was considering a larger Carradice type bag but am concerned about the back of my thighs slapping the bag and the bulkiness and increased size factor when the Brompton is in its folded state.

a spacer added between bag and seatpost is easy to fabricate.. just need a longer strap

you might just get a piece of plastic PVC pipe. run the strap thru it.

Or maybe 2 , one to secure the stand off to the bag, #2 to strap the standoff piece to the seat post..
shopping only solutions?
Look at the carradice Bagman frame then.. it clamps onto your B17 saddle rails,
or the new standard Brompton saddle, if you don't make it so Pushed forward.

the saddle provides the bag loops.. but it will still be there sticking out back
when the rear is folded under , and the seatpost pushed down..

I'd just put those things in the Touring bag, up front, instead.
I use 1 rear pocket for that stuff, the other for water..

I have a Carradice "Nelson" on my Brompton and have no such concern. I wonder whether I need my rear rack anymore.

Never mind the tools, it fits a Six Pack perfectly .

I put the tube in a protective sleeve and stuck it in the main tube, rear portion; The spanner (small adjustable version) I wrapped in some thin sponge and stuck it in the main frame tube, front section. Standard patch kit, always carried with me on all bikes. Tool-less tyre removal technique, no lever needed. No bag needed for everyday use. Folded bike goes with me everywhere, no lock.

Don't find Brompton's tires a POC to take off, by hand only, myself.

Tool-less tire removal on a Brompton? really? Even with the tire levers I have difficulty, I cant imagine doing it tool-less. On 26 inch tires, no problem.

I would leave the bag cower in the bag. You do not need it permanently on the bike if you sometimes ride without the bag.

I have decided to get a mini framebag for the brompton myself. Decided that for the B that is better than a seatbag.

+1 on hiding stuff in frame and seattube.