So, unless you’re driving a car made in Liberia, Burma or the United States, you probably need metric fasteners. Although the United Kingdom uses the metric system for most official purposes, the use of the imperial system of measure, particularly for use at home, is widespread and is permitted by the law. Many sources also cite Liberia and Burma as the only other countries not to have done so. The metric system has been officially sanctioned for use in the United States since 1866, but it remains the only industrialized country that has not adopted the metric system as its official system of measurement. Metric vs Standard (Imperial) Measure From Wikipedia:

License Plate Screw Types BMW Rear License Plate Screw Comparison Left to right: BMW OEM zinc-plated steel pan-head screw, 316 stainless steel button-head screw, 316 stainless steel socket-head screw, black-oxide over 18-8 stainless steel pan-head screw, 18-8 stainless steel pan-head Torx screw, 316 stainless steel flat-head screw. Much of the information provided here comes from our experience with BMW automobiles, but the same general principles apply to any license plate hardware, for any automotive application.

It also briefly explores current trends in license plate brackets, plate “relocators,” and “tamper-resistant” fasteners. This guide attempts to take the mystery out of buying license plate screws and installing front and rear license plates. In most cases that means cheap zinc-plated steel screws with minimal corrosion resistance, and even less eye-appeal. Whether you’re driving a used $10,000 Toyota or a new $90,000 Porsche, chances are your license plates are attached to your car with the hardware that came with it. 21-pc slim line anti-theft stainless steel rear license plate screw kit for BMW Updated: Augby Phil Your car deserves better.

License plate mounting hardware