An 'Extra Light' Brompton includes the following titanium parts: Rear triangle, forks, seatpost, folding-pedal axle and wire mudguard [fender] stays. The rest remain standard and the frame is always steel.
You can, of course go further and use a titanium railed or carbon saddle and weeny out on other parts - cranks and the like as well as lose the odd gram by swapping out steel bolts for titanium or alloy. Heck you can even get ti spokes these days - but none of these are offered on the Brompton 'stock' and these would be your own aftermarket upgrades.
The major parts they do offer give the following weight savings (My steel figures are based on Merc weights not Brompton but are likely to be very closely comparable), which isn't a massive amount lighter when you factor in the frame - but it's good to remember that most people want a light Brompton not because they want a racing machine but because they want it to be easy to carry when folded.
Steel = 808g
Titanium = 455g
Saving = 353g (43.6%)
Steel = 610g
Saving = 345g (56.6%)
Steel = 475g
Titanium = 275g
Saving = 200g (42.1%)
Overall saving: +/- 898g
Merc frames are aluminium and a bit lighter than steel. (I don't have a figure on this weight saving but this is what I am doing with my project Merc/brompton hybrid: ti everything [forks, rear, seatpost, bollts], singlespeed, single brake setup with Ti Brooks B17, and no fenders, rack or any such paraphenalia)
I expect it to be rather light and easy to carry