zantopst - I'm guessing the high fares are down to the fact that the class says Premium Economy on the booking engine when you select economy so I'd guess that small error in the booking engine is the main reason for the exceptionally high prices. When I searched Nantes in July this error was not present and the prices were reasonable.
Also, I would expect flights around that time into SOU to be a little more expensive around the Olympics due to SOU's reasonable proximity and good rail links to centeral London.