Merkel is keen to enter a trade deal with the US, but Macron seems strongly opposed.
If the EU was to get a trade deal with the US, and Corbyn got his wish for a Customs Union with the EU, the US could use EU entry ports as a back door to tariff free imports into UK, while UK manufacturers would be subject to tariffs when exporting to the US.
That goes for any country with a trade deal with the EU by the way. Japan, Canada to name a few could use, say, Rotterdam to land their goods and then freight them to the UK tariff free.
Corbyn’s idea of a Customs Union with the EU would be bad for UK companies producing goods for export.
Better to have a clean break with the EU and take up Trump’s offer of a trade deal, while it’s still on the table.