At the German embassy you show your pension statement from the pensions authority, and they issue a nice looking certificate, stamped and signed by the consul, stating you get above 65000 a month income. What noone tells you until you submit it to immigration is that you need a 'verification' stamp on it from the Ministry of Foreign Affairs. Immigration will waive that for a (ridiculous and hefty) 'fee'.
Do the footwork instead guys.
Why the Brits or anyone else can't do that is beyond me.