Off course you can get a new Passport at the German Embassy while still registered in Germany, it cost only 171 Euros while the one for non registered costs 101 Euro. Info of German Embassy HP available.
In the past the Britain's went all over the world to conquered foreign Countries and have taken whatever they could get, now the people of this Countries rightful coming to Britain. It's just Karma.
To pay Taxes he needs to have the compulsory Insurance (Por Ror Bor). But as the Bike is older then 5 years he need to get the Inspection first, they can arrange the Insurance too at the same time.
I would consider to get a 5 to 8 Kwp Hybrid System in this case, you may add later a Battery as prices are still dropping. Additional ones the System is installed and you see the benefit you tend to use more power as it is free.