Are you sure, the small print reads…
Traders with no residence or habitual abode, registered office or business management in Germany, in another Member State of the European Union or in a a country where the Agreement on the European Economic Area applies, must provide the name of an authorised recipient in Germany when applying (section 123, Fiscal Code (Abgabenordnung)).
The implies to me that you need a address/agent in Germany or you cannot complete the form. UK is a member state of the EU, so therefore you need to supply a German address.
But i am baffled where we get this from! Any ideas?