Only in the one that calls for it. Understanding and Control can only be placed where they show up on your objectives chart.
There is no particular bonus to having control in Germany--surprisingly--except that it prevents Germany from placing any PFs there and it consequently loses the ability to use home base as a reserve.
Makes sense, thanks. Thematically it's weird that a British-controlled Germany can still act independently outside the Fatherland, but that's the only restriction I can see in the rules. The powers that want an understanding with Germany may wish to prevent British control in this scenario.