That's already happened it seems, here's Churchill's words spoken on 16th June 1940 and with full French agreement:
"The two governments declare that France and Great Britain shall no longer be two nations but one Franco-British Union. The constitution of the union will provide for joint organs of defence, foreign and financial and economic policy. Every citizen of France will enjoy immediately citizenship of Great Britain, every British citizen will become a citizen of France."
Not sure why no-one takes any notice of this fait accompli.
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It was never ratified as the Germans came en masse in the Traditional way and interrupted proceedings, though they did make an offer of their own.
If we joined with them we could keep our colonies,including even theirs we had stolen at the end of the First World War
After Dunkirk the public mood turned against the idea and seeing the way the wind was blowing suddenly as if in an Epiphany Fascism became far less acceptable to the Rothermere Press , King Edward and the Conservative Party.than the current EU arrangement is today , even though they had previously been big fans of AH and co.
A new surge of Patriotic Ardour was de Rigeur!
I suspect that what put them off the previous approach was the fear that the Public might use them for decorating Lamp posts with lengths of Rope.
Fortunately for the Country Churchill was wily enough to turn to Attlee and the Labour Party for support to prevent a surrender by the cowardly Conservatives, who had run out of Countries they could give to Hitler in the vain hope of buying "Peace in our Time" on a piece of toilet paper.
And when after Brexit the Proverbial hit's the fan, the same trusty instinct for self preservation will give rise to a clamour from the Press and Conservatives to rejoin the EU, (by which time they will have moved the money in their offshore Tax havens somewhere safer and sold off what little remains of this country's assets and infrastructure.)
History is fun, isn't it?
And the Wages of sin tucked for safety in an Offshore Tax Haven.