It throws a significant chunk of red (but vegan-friendly) meat to the vast number of Labour party members who want a soft Brexit or none at all, and it calms things down internally for team Corbyn after a hard week which saw the removal of the General Secretary Iain McNicol, provoking the ire of many of Corbyn's critics in the Parliamentary Labour Party and beyond.
The concept is old enough. Analysts say that both sides, the European Union and the United Kingdom government, have been continually throwing out accusations against each other which has created an atmosphere of concern for FX traders. Like a free-trade area, a customs union scraps internal tariffs and quotas, but it adds a common external tariff. The EU's customs union is essential to supply chains across Europe, with cars and cabbages freely criss-crossing borders many times over. Its adherents say that it facilitates free trade by ensuring all members charge the same import tariffs to countries outside the union.
Labour also say they wish to leave the European Union single market so that they can get on with state subsidies for investment in British industry and following our own public procurement policy (which might, for example, mandate Whitehall, nationalised enterprises and local authorities to buy British supplied goods and services). There are lengthy controls at the Turkish border with the EU.
European Union citizens arriving in Britain during a post-Brexit transition period will be able to apply for indefinite leave to stay in the country but their rights will be governed by British, not EU law, the government said on Wednesday.
But Mr Corbyn said his call was "consistent" with Labour's position since the referendum, and an "appeal to MPs of all parties" to "put the people's interests before ideological fantasies" to protect jobs, living standards and the economy and avoid a hard Irish border.
Alternatively, Britain can make a free trade agreement with the EU.
The EU wants a deal on Britain's future relations with the bloc agreed by October, so EU leaders can approve the whole deal at a summit and launch the ratification process in the 27 remaining EU members.More news: Florida high school survivors return to class
Some pro-EU ministers like Greg Clark want her to make a binding commitment to align Britain's rules and regulations in key industry sectors with those of the European single market.
Leaving the customs union is like "giving up a three-course meal for the promise of a packet of crisps", the former boss of Liam Fox's global trade department has warned.
"There's no bilateral trade deals in a customs union" - no Germany-China deals or Britain-U.S. deals.
The FT's Jim Pickard says that compromise will come when May uses her own big speech on Friday to try to placate the rebels by promising to replicate the benefits of a customs union through the creation of a "European traded goods area", which "sounds suspiciously like a form of customs union", notes Politico's Jack Blanchard.
Would the EU Trade Commissioners even agree to have (presumably) the British International Trade Secretary Berry Gardiner tagging along when they go to Canberra, Delhi, Beijing or Seoul?
"I'm concerned that time is short between now and next autumn", when any Brexit agreement must be concluded to allow time for parliaments to ratify it, he said.