yup, both UK cars and JDM cars should show negative figures.
One thing that never fails to surprise me (as a person who has worked in temperatures as low as -50C and as high as +50C) is that people can band temperatures so closely together.
For many folks, -10 to +5 feels the same with no actual sense of difference.
Also, wind chill factor will make it feel much colder than it actually is without causing the cars temp reading to drop.