I tried to buy one from Amazon but after testing it today the beam is clearly symmetrical and has no advantages over a normal bike light. I live in the UK so the StVZO regulation is not one which I have to comply with but I feel it is part of being a responsible road user. I'm open to the idea of having one that mount son my helmet.
If you live in UK, you should have no problem ordering from Germany. The mail order houses there sell mostly lights that are street legal in their home country, and label non-compliant lights as off road only.
EDIT: The above goes for bike-mounted lights. Helmet-mounted lights do not seem to be under the same regulation.
If you absolutely must buy from UK, you can look up models on German sites and see if they are available in UK. Although, UK and continental Europe seem to be strangely separate markets even if they are in the same customs union.
The StVO laws for bicycle lights only define the minimum Voltage and some attributes like: "The Rear Light should not be able to blink".
The StVO should be updated to use Lumen instead of Voltage among other changes, and this might happen soon with Light Manufactors lobbying for these changes.
However, the StVO says very little about the quality of a light, it's visibility for other road users or the ability to help you with vision while riding.
It would be wiser to look for a Light with a beam that does not glare in the direction of the opposite traffic, is visible from the sides and bright enough to light your way.
Reading reviews might help you more than relying on years old german legislation.
We might be known for our accuracy and efficiency but not for our reasonable traffic laws.