"* Longer wheel base also for lower position and less knee-banging when riding out of the saddle"
But is this a long wheelbase model or did you make some kind of modification to increase the the wheelbase?
The Brompton extended its wheelbase by an inch in late 2003, a few months after I got my first Brompton.
The current frame+Sbar produces the same geometry as a roadbike when comparing the grip area of Sbar with the top position of a drophandlebar, the Ergon bar-ends produces very similar position to hands facing forwards on the brake hoods of a drophandlebar. What you're lacking is the few inch lower position for downhills.
One issue I see in Brompton's P bar is the lower drops position for going down hill
and into a headwind, is there, but if you have to hit the brakes ,
you have to move your hands. UP to get to them ..
I'm not using P-bars for exactly that reason. S-bars is lower than M-bars but brakes right by hands. Even with hands on the bar-ends shown I can touch the levers a little.
I don't know why Brompton doesn't sell P-bar with a pair of brake levers connected, then I'd be interested, or someone make a kit, because even S-bars is higher than the dropped lower position on roadbikes.
I know some people who fit those arm-rests extensions.
B's M bars are fine by me, at my height. about goldilocks for an Old guy.
though I've seen pictures of substituting a Modolo Dumbo adjustable trekking bar
for M bars , on that stem, though some clamp bolt loosening
is definitely needed.. to fold down that bit of kit.
QBP has doubler parts to use 2 levers for one brake, so You could do that.
Brompton's designs seem pretty conservative, , think about it a long time
before making subtile changes ..
Hinge plates for example . from forged and hand fillet brazes
with a skilled person trained to do that job,
to A Cast hinge , added someone to machine each casting,
but the brazing is done with a torch array , and so more efficient
in parts produced per day.
a few jigs get the job done, and I presume de skill the jobs, a bit.