What if someone applies for a Divorce Petition in another country?
Once a Petition is issued in one of the EU Member states covered by Brussels Iia, the Courts of that country have exclusive jurisdiction. Any proceedings commenced in another Member State will therefore be stayed (halted). If proceedings are commenced in a non-Member State, then second proceedings are normally stayed, but this is not guaranteed.
Foreign decrees will not necessarily be binding on the courts in England and Wales. If the court does not recognise the foreign decree (a legal term meaning accept it is valid and binding in this jurisdiction), then you would need to obtain a decree of Divorce/Dissolution in this country, for the court to be able to deal with financial issues.