Really fascinating article on precisely this topic from Forbes, last year JJ that you might find informative.
http://www.forbes.com/sites/nadiaarumugam/2012/10/25/why-american-eggs-would-be-illegal-in-a-british-supermarket-and-vice-versa/
Opening paragraph from the article;
"Believe it or not, United States Department of Agriculture (USDA) graded eggs would be illegal if sold in the UK, or indeed anywhere in the European Union (EU). It’s all to do with the fact that commercial American eggs are federally required to be washed and sanitized before they reach the consumer. EU egg marketing laws, on the other hand, state that Class A eggs – those found on supermarkets shelves, must not be washed, or cleaned in any way.
“In Europe, the understanding is that this mandate actually encourages good husbandry on farms. It’s in the farmers’ best interests then to produce to cleanest eggs possible, as no one is going to buy their eggs if they’re dirty, ” explained Mark Williams, Chief Executive, British Egg Industry Council in a phone interview.
According to the USDA, despite how conscientious and thorough modern day farm-management practices might be, there will still be a small percentage of “dirty eggs” produced. Dirt almost always equates to chicken manure and, if the eggs are produced in a free-range system, other raw agricultural commodities that hens might pick up from roaming freely.
Any feces on the exterior of an egg shell poses a food safety threat from potential cross-contamination if, say, a consumer cracks open an egg then proceeds to prepare a salad with those same bacteria-riddled hands. Since egg shells are porous, there’s also the possibility of micro-organisms migrating inside the egg under certain conditions."