Fixed gear Brompton? Unless your using two brakes to slow, any pressure on the chain will just pull the tensioner out and therefore not slow you, will be a right old mess. Unless you add a lock to the rear end and don’t fold it, not going to be suitable!
Ti parts chainset and little bits, bb, hinge, rear sprocket etc.
H+h carbon post, some carbon bars and those super light brake levers (15-25£ online)
Velocity rims, hubSmith rear hub, Phil wood front.
To fork / triangle - rear I question whether Brompton made for a few reasons.
Stupid light SI carbon saddle
Stock 2015 frame, m type stem