Brompton steering geometry

[531colin]

...........
Both 72 deg head angle, tourer 2" for rake, Holdworth 2 1/2" fork rake. A had some 2 1/2" rake forks made for the tourer, problem solved.
.........., I prefer low trail,.........

Thats it, you have your whole answer already, I wouldn't bother at all with the Brompton geo.
That is pretty low trail, with a 72 degree head 45mm offset (about 1 3/4") is too twitchy for me. (I am usually riding unloaded, or with a saddlebag only) I doubt any "off the peg" tourer exists with that steering geometry. Another thing to think about; if you are going for a bigger tyre/more tread/lower pressure than your existing tourer & Holdsworth, then tyre drag will be greater; for the steering to "feel" the same, you might want a touch less trail. (bike steering is to some extent self-centering; the force is tyre drag, trail is the lever through which the force acts.)

Wheel flop is a function of trail; excessive wheel flop occurs when there is too much trail for that particular head angle.....although how much flop is "excessive" will also depend on the rider.. I'm not sure that wheel flop has a unit?** Wikipedia describes it as a "factor"

**....like millimertre or degree?

[rogerzilla]

Thorn expedition tourers are odd - very floppy steering, too quick by road racing standards. But they don't try to wag their tails when you honk up hills with 40lb of gear on the back, so they are well-designed for the job, even if not the best to ride unladen.