There are 2 issues here: 1) can I have black/silver plates at the rear and 2) can I use smaller-than-UK-legal digits so I can have a smaller plate.
1) Vehicles constructed (not registered, constructed) before 1.1.73 may display traditional style "black and white" plates i.e. white, silver or grey characters on a black plate. Lettering must either be 89mm or 79mm high.
2) Certain imported vehicles may be permitted to display number plates with smaller characters if a) the vehicle does not have EC Whole Vehicle Type Approval and b) the vehicles construction / design cannot accomodate standard size number plates. This lets you use 64mm heigh characters.
Remember, it is not the size of the plate itself that is set by the law, it is the font size and spacing that is set. If you have a short number (like mine) you don't have to have a plate that is long enough for a full size reg no.
If you want to be 100% legal, the smallest height for a traditional black/white plate with two lines is 11mm top margin + 79mm character + 11mm space + 79mm + 11mm bottom margin = 191mm or 7.5".
If your space is restricted (like in the rear bumper cutout for the JDM Z32) then that calc is 10 + 64 +10 + 64 + 10 = 158mm or 6.2"
Now a JDM plate is 13" x 6.5" (I believe other sizes too but not sure exactly) which is 330mm x 165mm so if you can justify the "cannot accommodate standard size" criterion, then you can legally (UK legal) have (and have MOT'ed) JDM size plates on your import (with the UK legal caption of who made them).
As it happens, if you use UK size letters on a JDM plate, that's NOT legal because your vertical spacing will be wrong.
Also, here is the place I get mine from
https://www.craigsplates.com/import-number-plates