Eu( including Britain for a few months) has signed a trade deal with Japan. Cars built in Japan will be cheaper as no WTO tariffs. After Brexit without a trade deal with the EU ( which will take about a thousand years to agree) no point in being in the UK.
Conversely, cars built in the UK will be more expensive, any not just by the 10% WTO. Which is why those 7000 are now nervous, along with the estimated 14000 who's jobs directly rely on Nissan in the UK.
That's why all manufacturers, not just automotive, are lobbying against a no deal exit. They are run by people bright enough to know that a no deal exit spells the end of their businesses, certainly in the UK.
But remember, we were promised the EU would roll over so we could have our cake and eat it, with us being such a significant insignificant rock in the North Sea. So fingers crossed, surely the EU will back down, give us an immediate trade deal, and also give us a £40b divorce settlement.