Ah I didn't realise the Ti seatpost has gone, Yes mine has that - why did it discontinue?
At that time Brompton had issues with reliable delivery of sufficient amounts of the seat posts from their supplier (I think in Russia) as well as prices going up. There had been a very tiny number of breaking seatposts as well as far as I know, but I don't think these were relevant for or cause of the decision. The idea was to use aluminium posts instead, gaining a roughly similar weight saving together with a cheaper price and more reliability regarding stock delivery from the supplier. This change happened in 2010, but after a very short while it turned out that the surface coating of the aluminium posts vanished quickly over time, leaving the seat posts in very ugly optics. Thus the aluminium posts were dished from the lineup after only one year or so and from thereon all Bromptons had the same steel posts, raising the weight of the ti-bikes.
Back then there were no 3rd party ti-posts or carbon posts (apart from the few Steve Parry built for some of his bikes and a small early series from an, I think, Brithish or Scottish small company, long out of production by now and I think also back in 2010). The first ones after 2010 came from Brompfication by Peter Griebel, who founded the company in 2010 or 2011 and they were for quite a long time the only suppliers of Ti-posts for the Brompton. There was an aluminium post from a french company, but it was quite heavy (and also long gone today) and another aluminium one popped up from Lightskon with integraded rear lights, still on the market today. It was only around 2016ish and later when the various Asian tuning parts companies established one after the other and only little later the various Chinese suppliers of cheap ti- and carbon posts and other tuning stuff hit the market and started to sell via
ebay, aliexpress and alike, initially partly with a somewhat dubious quality of some of their posts. Today there is a plenthora of offerings in all price and quality regions and under various brands and as far as I can judge there is not always direct correlation between quality and price. The only European brand currently selling a ti-post is I think Jtek. Worth noting that only since a short while there are offerings for a telescopic ti-post again but at a ridiculously high price.