According to the Evening Standard's 'Commons Whispers' Number 10 has rejected an application for an e-petition which called on Gordon Brown to "undertake to do whatever is necessary to prevent Tony Blair ever becoming President of the EU." The reason given by officials was that the request was "outside the remit or powers of the Prime Minister and Government."
Indeed. The Lisbon Treaty, which creates the post of EU President, states that he or she is to be appointed by the European Council acting by qualified majority. So no veto for Gordon over Tony, even if he wanted one.