they wont be any cheaper.
80% of brittish car components come from france germany and italy.
japanese cars are not beeing manufactured in the UK but in turkey (nissan, UK right now, soon as well turkey or hungary or romania), which is in the EEA, which means UK cant make deals with turkey, rest in hungary and romania.
brittish cars will be more expensive for brittish citizen, japanese cars will be more expensive for brittish citizen, european cars will be more expensive for brittish citizen. in the end brittish cars will increase in price the most, as parts imported to the UK get 10% tax on, and if you export the car another 10%, which means there will be less demmand for brittish cars in europe (biggest market for brittish cars) as they will be around 19% more expensive than right now. you can drive american cars, but id prefer a horse over american cars, makes less dirt and consumes less fuel.
or uk can devaluate its currency to parity with euro, or even less would be necesarry, to not have workers layed off and the export still at least a bit competitive.
this would mean, a 20%+ devalution of currency that the UK falls from 6th biggest economy in the world to number 10, 1 spot before russia, 2 behind italy.
currency devaluate thou makes food imports for the uk more expensive which means that inflation rises and the spending power of uk citizen falls drastically. tv phones computers imported from asia rise in price at least 20% +10% import tax under WTO rules= 32% food rises 32% medicine rises 32%
UK banks having no passport rights in EU which means they instantly lose 80% of their clients, unless they move headquarters into paris frankfurt madrid or dublin (and a lot of jobs put of london with them).
men the list goescon forever and my finger is lazy typing on phone today.
anyway to sum it up: enjoy the no-deal-brexit cake