So I emailed Brompton and they said the axle and planet cage assembly cannot be sold separately. They are only willing to supply the entire hub. They offered to sell it to me at a discount (exact discount not stated due to "exchange rates and duties") but will not sell it to me directly, only through an authorized dealer. I told them that I want to install it myself but they refuse to send it to me due to the requirement for "specialist tools".

This doesn't sound right to me. I own the bike. It's my right to fix it myself, is it not? Didn't BMW and other car companies get their wrists slapped for this kind of approach to servicing their vehicles?