A senior executive from Nissan came to my son's school to talk to the A level Economics class about the company, particularly logistics. Somebody asked about the effect of leaving Europe. The Nissan guy wasn't going to give a group of A level students a straight answer, but apparently he definitely hinted that it would make logistics more difficult.
Currently the European business is totally integrated. If a customer orders a car, it's more or less made to order, which avoids having to stockpile completed cars. If one plant is working to capacity, production can be switched within a day from Sunderland to Spain and vice versa. This flexibility, without having to bother about cross border tariffs, is a huge factor in Nissan's success. Most of the cars are exported to mainland Europe.
Nissan has 7,000 employees in its Sunderland factory and over 25,000 in its UK supply chain. Those jobs are at risk if Nissan decided to move to another location.
Ford, Vauxhall and Jaguar Landrover could very well decide to relocate production. UK car manufacturing currently has an annual turnover of over £50 billion, generating about £26 billion exports. All the main auto companies are foreign-owned. Leaving the EU could very well be a disaster for the industry and the people employed in the factories and supply chains.
Pharmaceutical and hi-tech companies (also mainly foreign-owned) could also decide to relocate to mainland Europe. They already employ many researchers from EU countries and attract some of the best technical and scientific minds to the UK. Microsft, which has its European Research HQ in Cambridge, has already stated that the UK's belonging to the EU was a factor in its decision to set up its HQ in the city. From an American perspective, the UK is in the unique situation of being English-speaking and in the EU.
Astra Zeneca has its own airline flying employees between Cambridge Airport and Sweden. The workers are effectively daily commuters, but any barriers, such as work visas and delays at passport control, could mean that it relocates.
Unless BREXITers actually come up with an alternative strategy, my feeling is that the UK would be foolish to throw away the advantage which the EU gives us.