Rules for British citizens
Since 21 January 2011, the National ID Card issued by the United Kingdom for British citizens is no longer a valid document nor will it be valid for travel within Europe.
All British nationals holding the national document were identified and it ceased to be legally valid to use this document for travel in Europe. All citizens were duly notified immediately after the enactment of the 2010 law. Border agencies and other interested parties were also notified.
You can find out more about the subject here.