The EU Treaties provide for a specific mechanism for a Member State to leave the EU. The UK would remain subject to that procedure after a vote to leave the EU.
In the absence of earlier agreement with the EU institutions and the remaining Member States, the UK would remain subject to EU law for at least 2 years while the terms of its departure from the EU were negotiated, including the provisions in respect of the jurisdiction of the ECJ and the role of the EU in relation to international trade.
In the event that a negotiated agreement was not reached within 2 years, the remaining Member States would determine the terms on which the UK left the EU.
Were the UK Parliament to amend the 1972 Act in a way that purported to derogate from EU law during the period before the UK left the EU, that would be a potentially serious breach of international law.