Just a correction: England is NOT a sovereign nation and hasn’t been since 1707, nor would it be if it left the EU, it is part of a United Kingdom and sovereignty is shared between England, Scotland, Wales and Northern Ireland.
well, England would become a sovereign nation if it voted out — because the Scots would ask for a new referendum and leave the UK. The Welsh would follow and NI would be split between a large number of people who don’t want a border with the Republic of Ireland