Hmmm, I wonder why?
Proud managers describe 2m components "flowing like water" to the factory line every working day. Some orders from EU suppliers arrive within five to 24 hours; others, such as customised car seats, are summoned from local suppliers just 75 minutes before use. Not a minute is wasted.
Honda now fears that the border checks that could be introduced as a result of Brexit will clog up the process. If Britain were to leave the customs union, Honda estimates European parts will take a minimum of two to three days to reach the plant, and possibly as long as nine days. Delivery times of finished cars may be just as unpredictable.
To a car industry famed for its clockwork tempo, the potential delays pose an existential challenge. A warehouse capable of holding nine days' worth of Honda stock would need to be roughly 300,000 sq m — one of the largest buildings on earth. Its floorspace would be equivalent to 42 football pitches, almost three times Amazon's main US distribution centre. And its cost to operate would be as eye-catching as its proportions.
https://www.cnbc.com/2018/06/27/honda-faces-the-real-cost-of-brexit-in-a-former-spitfire-plant.html
"From an administrative point of view, we'd probably be looking at 60,000-odd additional bits of documentation we'd have to provide to get product to and from Europe"
"If we end up with WTO tariffs, we'd have something like 10% of costs in addition on products shipped back into Europe and that would certainly run into tens of millions. And likewise, when we're looking at components coming the other way, again tens of millions in tariffs potentially coming into the UK.
"That impacts our productivity, certainly in terms of the flow of product, but also it does hit potentially our competitiveness. Of course if we are shipping and competing against a European manufacturer in Europe, they're not incurring those tariffs."
https://www.theguardian.com/business/2018/sep/18/honda-no-deal-brexit-tariffs-swindon