Assuming you have a W212 E-Class or a W221 S-Class and you bought it within the last year, you can go back to your dealer and tell him that if he doesn't reprogram your sat nav to take UK postcodes you want him to return the money you paid for it as an extra. The argument is that a £200 Tom Tom or Garmin will take UK 7-digit postcodes, but a £2,000 Mercedes-Benz system won't because someone at the factory forgot to programme it for the UK. This makes the car a misbuild and clearly "not of satisfactory quality". If it's a C-Class, or an Audi, nothing can be done because 7-digit postcoding cannot be post-programmed into the systems.