EU law requires all member states to admit anyone who can prove their EU citizenship by producing either a passport or a National ID card from any member state.
So anyone in possession of a valid UK ID card will be able to use it for travel within the EU (or to those countries which are outside the EU but inside the EEA. i.e. Switzerland, Norway and Iceland).
Eventually it might be possible to obtain a National ID card without possessing a passport. However the current pilot scheme (which is solely for residents or workers in north-west England or for 16-24 year olds living in London)) is only open to those who already hold a valid UK passport (or, apparently, a UK passport which has expired but which was still valid on 1st January last year). See here:
http://idsmart.direct.gov.uk/index.html
Chris