Just googled it - Honda said that a 15 minute customs delay could cost them up to £850k a year. I have heard that stat before but I don’t fully understand what they mean by that.
From a logistical and supply chain perspective, they’d have to drop JIT and build some warehouses. Not the end of the world in the long term but a short-term problem, probably enough to make Britain a less attractive base for production. Obviously this is something that will be negotiated into a deal.
I don’t think you should neglect to reflect on the fact that with the EU-Japan trade deal in place (no tariffs on Jap car imports) Britain is already massively less competitive, that commitment to EV production is changing the landscape for the car industry, and that Brexit gives these companies a convenient way out.