Is it possible for the British parliament to repeal the European Communities Act 1972 without the consent of the European Union?

Yes it is possible, however this would be a very messy way to withdraw from the European Union. Taking such action without first informing the European Union would be seen in a very negative light internationally and would ruin the United Kingdoms reputation in international politics.

The proposed constitutional reform treaty did set out a proper method for the withdrawal of a member state.

A member state which withdraws from the European Union will lose all influence within the European Union and will be subject to all the same entry requirements as a new applicant state should it wish to join at a later date. Including the adoption of the Euro single currency.