There is no legal procedure for a country to leave the EUssR.
If Brussels said NO, would the British Parliament really defy it?
The vote of UK nationals today may not matter even they do vote to “Brexit.”
And I’m on record as affirming national sovereignty.
Strictly speaking I suppose you could be correct.But in practical terms the British Parliament could do things like stop paying the membership dues,refuse to allow people of certain nationalities to enter the country,levy tariffs on certain imported items that weren't previously taxed.If Parliament attempted to do any of those things how would Brussels prevent it? An armada?
All Britain needs to do is pass an Act in its own Parliament to leave. The EU cannot block an Act of the UK Parliament - not yet, anyway. Part of the reason for getting out is one day that might change.
If Brussels said NO, would the British Parliament really defy it?
Yes - and, if that happened suddenly in the wake of a 'Leave' victory in the referendum a lot of MPs who are pro-Europe would change their mind, because that would be such an overreach, it would be intolerable. It would make Brexit easier, not harder.
Oh yes there is a legal way to leave the EU. According to Article 50 in the Lisbon treaty: 1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.