Could England (plus its two lap dogs Wales and Cornwall) leave the EU but the rest of the UK, Scotland and Northern Ireland, stay?
How's this for a plan? Loath as I am to say it, perhaps our only option is the creation of an English parliament (England and Cornwall). This would be created as a part of a settlement for the UK as a whole with fully federal parliaments in Wales, Scotland and NI. Each parliament would have a large degree of autonomy including over things such as EU membership.
There is already an almost precedent with the UK's protectorates and dependencies such as the Isle of Mann, Jersey and Guernsey. Currently, under the dominion of the British Crown / UK government, these territories are NOT part of the European Union. Could an England (and Wales) within a federal UK be given a similar status - a state in the UK federation but outside the EU?
At a later time such a system would allow member states of the federation to either leave or join the EU with greater ease. Aspiring territories such as Cornwall would also be free to hold a referendum on becoming a fully fledged state within the UK federation - in or out of the EU.
Come on then - feel free to shoot my plan full of holes now.