The British Foreign and Commomwealth Office has published advice for British Nationals living in Japan. This includes contact numbers for the Tokyo Embassy and Osaka Consulate which it encourages those living in affected areas to call to confirm that you are safe.
The advice is available here:
UPDATE: 12:00 GMT - The FCO has issued an earthquake helpline for families of those in the affected areas of Japan. The number is 020 7008 0000