If England (with Wales and Northern Ireland) and Scotland are to re-negotiate their relationship, a Treaty of Disunion will be needed to replace the Treaty of Union which preceded the Acts of Union.

Just as in 1706-07 the negotiations may be a bit difficult, particularly on financial issues, but at the end of the process everybody should still be on reasonably good terms. I am sure England would not cause difficulties about Scotland joining the EU, because if it was not a member sorting out the consequences of disunion would be more difficult for England.

With both parts of the former UK in the EU, there would be minimal economic disruption or problems about the free movement of people. A resumption of the endemic lawlessness of the borders and the periodic wars, which characterised the Anglo Scottish relationship before the union (and indeed for a time after it with the two  Jacobite risings of the eighteenth century) would be in no ones interests.