hey my friend
this upgrade is better if you have the 2 speed brompton or the 6speed, because all derrailer related stuff is already there.

this is the sturmey archer 10 speed hub, it fit flawless in brompton.
it used as sprocket the tradicional 9 spline cassete fror 9 speed



sturmey archer 10 speed

you can buy here:
http://www.brommieplus.com/portal_c1_cnt_page.php?owner_num=c1_343119&button_num=c1&folder_id=44131&cnt_id=338800

you can read how it fits or feels herr:
http://xtracycleleap.com/10-speed-brompton-upgrade/