Bromptons

Titanium aren't always the lightest, a triple butted titanium frame for example weight 1400kg, but cost over a thousand quid, whether a normal 7005 alu frame weight only 100kg more and a lots cheaper, even £300 (spes langster's frame weight that much).
the reason people went for titanium is because of the ride quality, whether alu is stiffer.
an alu seatpost will be just as light as titanium, frankly, a titanium seatpost isn't necessary on the brompton.

I never said titanium was lighter...

Anyway I don't disagree about what you say on Titanium vs Aluminium...
What disappointed me was the cheapness feel about it... Aluminium is cheap and common. Titanium is un-necessary and therefore paramount, imperious...
Bromptons are beautiful objects, they like nice things... nice carradice front carriers, brooks saddles...
Otherwise, you might as well get a Merc instead of a Brompton, cheaper, lighter, kind of do the same job...
Szia

Loic