The Kingdom of the Netherlands consists of several countries and a few overseas municipalities. Only one of them, the eponymous one sitting on the northwest shore of the European peninsula, is part of the E.U. The rest are overseas territories, including the special municipalities.
So lets look at those special municipalities and the E.U. They consist of the islands of Bonaire, St Eustatius and Saba. Dutch citizens on those islands vote in Dutch national elections. But they do not live in the European Union. What does that mean?
First, there are border controls between the special municipalities and the European Netherlands. Land at Schipol, go through customs and immigration just like any European citizen returning from a foreign country.
Second, they are not in the single market. They can export duty-free to the E.U. (see Article 43) but with limits: (1) Re-exports can only be sent duty-free if the overseas territory imposes trade taxes at least as high as European tariffs (Article 45); and (2) services are not included (Article 51).
So all Parliament needs to do is declare England to be an overseas territory of the United Kingdom! The English could still vote for Parliament. In fact, they would still be E.U. citizens, strictly speaking. But Parliament could restrict non-British (or even non-English) from working in England.
Of course, England-based financial firms would no longer be able to sell financial services across Europe ... but British firms based in Scotland certainly would. Moreover, the U.K. would remain an E.U. member, and thus retain its voice in Brussels debates over financial regulation. As my GWU colleague, Christopher Mitchell, has pointed out in the Washington Post, retaining that voice is vital for the sector to thrive.
Now, I do not want to minimize the problems: other than retaining duty-free access to Europe, English manufacturing firms would face all the same non-tariff barriers faced by, say, Turkish companies. The dislocation of having financial firms move to Scotland is not nothing. And you would need to institute a customs boundary along the line between England and Scotland.
But it keeps Scotland in both Unions and keeps Britain at the European table, although its voting weight would drop considerably, given that English residents would no longer count.
There might be a problem with doing this unilaterally, however. Annex II of the consolidated Treaty on the Functioning of the European Union explicitly lists the overseas territories; London may not be able to add to them unilaterally. But it would be a legal mess if it tried; I have no idea how the European Court would rule.
Somehow I do not think this will happen, but I am not sure why I do not think it will happen. After all, something much weirder just did happen.