I disagree - while England most likely will never be great again due to the change in circumstances (for the same reason there will not be a renewed Roman Empire nor a renewed Asshur or Mittani or Prussia)
While that is there, the act of England leaving the EU will hasten the UK's and England's decline -- the British Union will end and most of the industries will move from England
The British union will not neccesarily end, not if people draw the correct conclusions about the over centralisation of power. If we reduce London's dominance, the SNP can be contained.
I'm not worried about the move of industries. That's already mostly happened. Industry makes up less than 20% of Britains GDP these days.