http://www.allboxerinfo.com/boxer-breed-types

The UK Boxer
What is the difference between a German Boxer and an American Boxer? How about a UK Boxer? The answer is very little. All are considered to be 1 breed: The Boxer.
However, each of these 3 Boxers have very slight differences. Dogs of these countries are bred to perfect these slight variances.
This UK Boxer fits the UK Standard precisely: Small feet with high knuckles, considered to be very elegant.

The German Boxer
The biggest difference between the German Boxer and the American or UK Boxer is bone size.
As you can see on this champion German Boxer, the bone structure is slightly larger than the American or UK Boxers.
With German Boxers, the nose is generally smaller; however only by a very small degree. The width of the thigh is slightly wider than the other types.

The American Boxer
The most noticeable difference between an American Boxer and the German Boxer or UK Boxer is the tight, shiny skin and coat. As you can see, the skin (other than the head) has no wrinkles. Most, but not all, will have a tighter coat.
The American Boxer, as well as the German Boxer stands differently than the UK Boxer due to the shape of the paws, knuckles and ankles.
Generally the muzzle and nose on the American Boxer is wider than the German or UK Boxer.