It's actually not so much an AmE / BrE thing...
Proxy is the lowest level of authorization to sign for a company or make legally binding statements. Thus, "Handlungsvollmacht" is the closest thing to proxy, whether US or UK.
Now, say you want to sign a paper on somebody's behalf - you would sign it"pp", which stands for "per procuration" (German "per Prokura"), even though hardly anybody knows this, and even though the German "Prokura" is much more than a "Handlungsvollmacht", and much more than a one-off signing authority on somebody's behalf.
Procuration is indeed used more in the sense of "Zuhaelterei", and you should avoid it (as Juliane had to find out the hard way).
And if you're trying to say that you are a "Prokurist", you should stick to Claudia F.'s version "power of attorney", that's as close as it gets to the "Prokura".
However, as LEO is a dictionary I am of the opinion that it should stick to the correct version, but should also mention the other meaning.