Done it.
You need:
New style brake levers, one with the derailleur shifter. The old trigger is no longer sold, so you have to buy new brake levers even though you don't really want to.
Post-2017 cable anchorage. It's a Heath Robinson affair, worse than the old type, and there are no instructions available. Find a good photo of an installed one.
Derailleur chain tensioner. You MAY just be able to swap the jockey wheels, which are sold as spares (I haven't tried).
Chain tensioner nut and washer. Different thread to your old 3-speed.
Actuator plate assembly.
Cable. The S-type one needs an inch chopping out of it to make it lie nicely. And yes, I did buy the correct one.
Brompton 1/2 speed freehub (and everything else to build a wheel).
Sprocket set. The roll-your-own approach is fine but I reckon < 15T for the large sprocket would cause the chain to hang up on the dork disc, and < 12T for the small one isn't possible. I used a Shimano HG 15T and a ZTTO 13T from eBay, plus a 1mm spacer between them. It works well. You have to buy the set anyway to get the dork disc and circlip.
3/32" chain. I used a SRAM 8-speed with a silver Powerlink. 102 half-links for 52 x 15.
Finally, you need to chop an inch out of the front brake cable as the new lever hangs lower. Very important if you have no mudguards. The rear brake cable will be fine.
There is a modest weight saving over the 3-speed, but nothing spectacular; I reckon under half a pound, but my 3-planet AM internals are probably lighter than an SRF3/BSR/BWR hub. Bit disappointed with that. The derailleur is probably a little more efficient, since the hub gear has chain tensioner losses too. The Brompton derailleur gearchange is agricultural but fairly idiot-proof as you won't damage anything if you shift when not pedalling.
Cost of conversion was about £120 but I had the hub and the rim already. Will report back after some proper mileage.
