Thanks everyone for the feedback.

Fitting a cassette is not an issue, since the goal is precisely to remove the two-cog cassette + derailleur and replace the three-speed Sturmer-Archer hub with a Shimano geared hub.

It appears that the Nexus 8 needs 132,5mm while the Alfine 8/11 require 135mm (Source). The eight-speed Sturmey-Archer gear hub has a bad reputation, which is the reason I'd rather go Shimano.

Brompton seems to use HiTen steel, and I guess it's brazing not welding.

incidently, Juliane Neuss in Germany has been cold setting Bromptons for years, but it seems only doable right by someone experienced with Bromptons.

I might need to change the bottom bracket to get a correct chainline.