Well if that option is not available each company will have to weigh up a significant number of variables in order to ascertain the best way forwards
Eg what will any increased costs actually be
Can the get the UK government to ammend the tax codes for example to offset any additional cost as tax deductible?
What would be the cost to move operations to a new country... would you sell your existing factory knowing a competitor could use it and the local skill set to quickly challenge.
And thousands of other variables
And whilst the government of Japan rightly voices the car industries concerns they also have the likes of Hitachi pushing for tens of billions of pounds worth of contracts to build nuclear reactors for the UK...
It won't be full on doom and gloom and neither will it be rule Britania... probably a very bumpy ride for something not dramatically different to the status quo though as the smaller party the odds are we end up holding the shitty(ish) end of the stick
Also if you take Honda for example they have produced 74413 cars in Europe this year so far and 2,855,654 worldwide
Whilst 2.6% is not irelavent it's not going to be their primary focus (figures from Honda plc monthly financial statement August 2016)
http://world.honda.com/investors/financial_data/monthly/