ID cards have always been acceptable for travel within the EU. The Uk used to issue "British vistors passport at Post Offices for the purpose but stopped doing so some time ago.
From -
http://europa.eu.int/youreurope/nav/....html#12709_10
Special circumstances pertaining to the United Kingdom and Ireland
EU citizens travelling from a Schengen State to the United Kingdom or Ireland or vice versa may be subjected to checks. However, such checks may not go beyond the provisions of Community legislation in force. These borders may therefore be crossed simply upon production of a valid identity card or passport. In principle, no question may be asked as to the purpose of your journey, your means of subsistence, etc.
Passport checks in the case of members of your family who are not nationals of a Member State of the European Union may, where applicable, be aimed at establishing whether a visa has been issued and their status as a member of the family of a national of an EU Member State, i.e. the status that carries with it the right to enter the territory in question.
On the other hand, under the terms of the common travel area, there are in principle no longer any checks on persons travelling between the United Kingdom and Ireland.