As confusing as it is, dog foods labelled as 'for working dogs' are not always just for working dogs. Many manufacturers add the phrase simply because, by doing so, the food is automatically exempt from VAT which knocks a whopping 20% off of the retail price without effecting their profits at all. 'Breeder bags', 'assistance dog foods' and foods for 'resting working dogs' are also VAT free.
There are also no strict rules over what makes a 'working dog food' so any manufacturer can say that virtually any of their foods are for working dogs. In fact, while you might expect the protein and fat levels of working dog foods to be higher to allow for their increased activity, you will find that many 'working dog foods' actually have fairly low protein levels and aren't significantly higher in calories than standard dog foods.
So why don't all manufacturers just label all of their foods as working dog formulas and save their customers the VAT? Simply because many owners won't feed a 'working dog food' to a non-working dog. As a compromise, some companies produce two almost identical versions of the same food, one for the mainstream market and one VAT-free 'working dog' variety for more savvy customers.