As, by May And June 2005 the constitution was reject by Netherlands and France, the whole ratification process was brought to an end. To stil go ahead with the referendum would have been a pointless waste of money as by that point the result wou;ld have bean irrelavant.
Labour (Blair, Brown, and their manifesto) promised a referendum on the 'constitution'. The constitution was voted down by other countries, and re-introduced as a revised treaty. Labour signed up to that as it wasn't the 'constitution' that they had promised a referendum, although it had most, if not all, of the same effects.