Taiwan, as far as I know. I'm trying to set up production of titanium rear frames and forks with standard hub spacing for Bromptons in the next month or two with an aerospace company here in Russia. They were approached by Brompton earlier about the components, but ended up losing to Taiwan. Or China. I doubt that it's Mainland though, QA and legal would be a nightmare to enforce, given Brompton's attitude towards the chinese market in general.

As far as full on titanium goes, it'll be crazy expensive to manufacture anywhere in the UK. And making it elsewhere would go against their mission/marketing agenda/etc. And if I make one in Russia by myself and try to sell it in the EU or US, I'll be a felon, pretty much.