It s not just the fork you need to upgrade, you need a strong headset too and the Brompton OEM bearing are unlikely strong enough to handle the added cantilever load.
to convert to front disc brake (especially hydraulic with 160mm disc), you would be a 16" fork with 100mm hub space that follow this design:
https://www.planetx.co.uk/i/q/FOOOCR...fork-disc-only
Or a Aluminium Hydro fork or carbon (not cheap or easy to hand craft)

and you would need to fit something like a BMX headset

Basically do what tern did with the Verge x11 or get Ben Cooper's kit: https://www.kinetics-online.co.uk/fo...n-disc-brakes/