I think this is only true for EEA member states that have adopted the Schengen Agreement...
(http://en.wikipedia.org/wiki/Schengen_Agreement)
Which the UK has not. What you say is completely true if travelling from, for example, Sweden to Spain. But travelling from the UK to anywhere or from anywhere to the UK requires a valid passport as I understand it.
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Schengen agreement
I think this is only true for EEA member states that have adopted the Schengen Agreement...
(http://en.wikipedia.org/wiki/Schengen_Agreement)
Which the UK has not. What you say is completely true if travelling from, for example, Sweden to Spain. But travelling from the UK to anywhere or from anywhere to the UK requires a valid passport as I understand it.