I ordered Brompton + 8GearSturmey Archer from Kinetics and awaiting it. It is ca. 220,- english pound - the hub incl. building wheel etc. (ordering the 1gear AlaCart from brompton saves money). If one already has the bike, its probably not an opportunity: as far as I know, Kinetics only does the mods on a new bike ordered from him. Ben Cooper has been very friendly and helpful and has a good reputation for doing things with bikes. A weakness might be (according to users in the brompton talk): he sometimes needs a long time to deliver and doesn't always answers to emails.

In Berlin Germany you can also get the Sturmey Archer swap for your old/existing brompton. For conversion they charge 333,- Euro. The hub is modified a little and the brompton derailer, but not the frame. I believe, the shop "faltrad-director" also does postal orderings. The owner Christoph Beck is said to be a specialist for folding bicycles. (e.g. seem to be well liked by the birdy-people and has worked on the construction of pacy-bike)

www.faltrad-direktor.de/faltrad%20-%20typen/index.html

If you are willing to broaden the triangle/drop outs and are gifted to do it yourself, you can get a conversion-kit from Juliane Neuss. A rear wheel with the 8-Gear Premium hub with detailed explanation to prepare the triangle. I don't know the price for that. If she mounts it, it will be about 458,-Euro (delivery in Germany, so probably more to your place)
see also
http://bommy.exblog.jp/m2008-01-01/
Hope that helps.
Somnatash