According to El Reg the

According to El Reg the next generation of biometric passports will require your photograph of you to be not smiling. This is further backed up by the guidelines on the UK Passport Office website:

• Applicants must submit two identical photographs, which have been taken in the last six months.
  
• The photos should be printed on normal photographic paper and should be 45 mm x 35 mm in size.
• The photo should show a close up of the applicant’s head and shoulders so that their face covers 70-80% of the photograph.
  
• It should be taken against a white, cream or light grey plain
  background so that the applicant’s features are clearly distinguishable
  against the background.
  
• The photo must be of the applicant on their own, with no
  other people visible. It must show their full face, looking straight at
  the camera, with a neutral expression, with their mouth closed.
  
• Photographs must be printed at 1200 dots per inch resolution or better if they are digital or scanned photos.