The titanium fork and rear triangle have been manufactured at several places since they exist, including Russia.
Brompton had several times quality and delivery problems with these two parts that affected the availability of the Superlight and CHPT3.
But from what I read, the current titanium fork and rear triangle are made in UK by a joint venture company between Brompton and a UK titanium parts specialized company.
The full titanium frame could be built by the same company in the UK.