UK border controls on January 1, 2021: the death knell for UK vehicle production? 

During the Brexit referendum, then under Theresa May's premiership, and also during the early days of Boris Johnson's tenure, the UK was either "promised" frictionless trade or led to be believe imports, especially, would face no specific additional burdens.  Any additional friction would be as a result of the EU, implying that it would be exports which would be hit.  Indeed, much of the discussion in last year's 'no-deal' planning suggests that there would be both zero tariffs on most imports and only light touch import procedures, with 'easements' for deferring VAT payments for example.