There's huge questions as to whether that freedom and flexibility could be maintained if Scotland was an EU member in the CU/SM and England wasn't.
It's not an issue. Norway/Sweden or the Swiss/EU borders show how it can be done. Essentially:
- Immigration controls continue to be carried out on the border of the CTA, just as now.
- Customs controls exist as on the Norway/EU and Swiss/EU borders. There aren't actually that many border crossings between Scotland and England, so it's not that much of an issue to erect customs posts at each one.
- Customs controls are carried out on the train for those stopping between Edinburgh/Glasgow and the border, while those proceeding directly to/from Edinburgh/Glasgow will have customs posts in the railway stations, manned by both Scottish and English customs authorities. It might mean that trains will tend to run non-stop between the border and those cities, but it's not a big deal.
- Most crossings are in the middle of nowhere, so pedestrians are a moot issue.
- Flying is dealt with in the same way as Irish/Channel Island/Manx arrivals today - they bypass passport control and go through customs.
An English Parliament could not be a copy/paste of the Scottish-they won't buy it at all. Plus, London couldn't be a part of it anyway.
But why not? I always imagined this:
- England takes over the House of Commons, and it becomes the seat of the new English parliament.
- The House of Lords becomes the new Federal Parliament, in which the English MFP's total the same as the Scottish/Welsh/NI MFP's. As a result, England cannot force any legislation through the Federal Parliament without the explicit consent of at least one rUK MFP. Given that the Federal Parliament is likely to have a diverse set of elected representatives, it would become a matter of negotiation between the countries to decide on UK-level policy.
- Devo-Max for all four countries, with the provision that Northern Ireland remains under Federal supervision. If the Northern Irish Assembly is suspended, then the new Federal Government takes responsibility for Northern Irish affairs.
- The Federal Government itself is on the Swiss model - the biggest party from each nation is entitled to appoint four cabinet members. Effectively, it's a permanent coalition to ensure stability.
- Clear division between national and Federal competences.
I don't see why England couldn't have her own parliament. EVEL only works as long as the Tories have a majority in England and in the UK - if the next government is a Labour/SNP coalition, then EVEL is out of the window as Labour would probably need Scottish votes to pass English legislation.