If England and Wales leave the UK political union Scotland and Northern Ireland should be able to retain their membership as the remaining UK entity. The vote must take place as soon as possible in order to allow provision for Scotland and Northern Ireland to remain in the EU before Brexit.
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As per European Court of Justice ruling Article 50 could be unilaterally revoked by the remainder of the UK, Scotland and Northern Ireland, who would be the Continuing Member State.
England and Wales would be free to deliver Brexit without the need for a backstop arrangement.