But that is EXACTLY what Corbyn is advocating.
He wants a "Customs Union" with the EU:
"The Labour party now supports Britain remaining in a customs union after Brexit."
https://www.theguardian.com/commentisfree/2018/feb/27/customs-union-brexit-european-union-eu-turkey
"Intrinsic to the idea of a customs union is that goods cannot enter into it without paying the agreed common external tariff. Therefore only the European commission, and not individual countries, can negotiate trade deals with third countries. So Britain would not be able to negotiate independent trade deals with third countries that involved the lowering of tariffs. That would remove one of the principal advantages of Brexit: the opportunity to secure lower prices for consumers by importing goods tariff-free from outside the EU.
"In addition, when the EU negotiates a trade deal with third countries, the deal would open up British markets to these countries; but it would not open up their markets to Britain, since Britain would not be a member of the EU.
A customs-union Britain would therefore have to conclude separate trade agreements with third countries so as to secure the benefits of the EU deal. But there would be little incentive for third countries to conclude such agreements, since their goods would already be able freely to enter Britain."
"So Britain would have to align itself not only with EU trade policy but also with the laws of the EU in certain areas: not only as they are now but also as they may be altered in years to come except without a vote to help decide on them. Britain would be subject to regulation without representation"
And there would still be "friction" at the border:
"The EU has so far agreed to frictionless trade and open-access road transport only for countries that accept free movement members of the European Economic Area (Norway, Iceland and Liechtenstein) and Switzerland, which has bilateral agreements."
THAT ^^^ is what Corbyn is advocating. Something you called a non-starter. Agreed. It's never going to happen. And it would be awful if it did. Almost as awful as what May wants. Which the EU has already rejected.
There's NO SUCH THING as full access to EU markets, without being in the EU. That is a fiction. And a great example of the thinking inside the Corbyn fact free zone.
So, again again, off the cliff edge.