The titanium posts I've seen have looked fairly tatty with scratches and scuffs.
If Brompton is also having issues sourcing them then it seems like a good idea to replace with an aluminium post. Hopefully this will prove light, cheap and strong enough compared with the titanium model.
Maybe if all goes well with this trial the alloy post will become a standard item on even the non-superlight Bromptons (most other folding bikes at this price don't have a steel seatpost... but I'm now straying into stating the obvious)