More to the point, it looks as if the reflector on TLAU100 is far from parallel with the rear casing, so when mounted on a rack or any other vertical surface, the reflector (and possibly also the light itself) won't in fact conform to UK CUR. The first review on this page seems to confirm that.
Hmm, I'm not sure I get you (or that review). As far as I know, the AU100 comes with a fairly standard CatEye mount, which will probably best fit a seat post. The reflector should be broadly vertical, because then the emitters will also be horizontal and pointing directly rearward. The clamp may also fit on the seatstay (may need some packing out), but the two parts of the mount can be rotated relative to each other to accommodate that.
You can also buy a CatEye clamp for the rear of a rack, which will adjust to suit many racks, and other bits to suit different thickness of tubes etc.
The reflector doesn't need to be all that vertical, since it's a retro-reflector, and will reflect the light back towards it's source for most orientations of the reflector and the light, as long as these aren't too extreme.