Just passed a silver Mercedes parked outside Asda on Beltonfield Road. We're on route to a callout at the Royal Infirmary. Can't stop, but the reg was a KS something.

REGistration

License plate. Here they’re talking about a registration number for the vehicle.

А что это за две разных plates?
В UK только одна - registration (reg) / number plate

British English: https://books.google.com/ngrams/graph?content=license+plate%2Cregistration+plate%2Cnumber+plate&year_start=1800&year_end=2019&corpus=29&smoothing=3

I’ve heard British people call them license plates too. The number on the license/number plate is the vehicle registration number.

I don't think so. What licence?
Even new cars are usually sold pre-registered and the same plate remains if you sell your car.
https://www.gov.uk/get-vehicle-information-from-dvla
https://www.gov.uk/driving-eyesight-rules
https://www.regtransfers.co.uk/number-plate-guides/car-registration-plates

However, I just realised how strange the name of the government body is - Driver and Vehicle Licensing Agency - https://en.wikipedia.org/wiki/Driver_and_Vehicle_Licensing_Agency

It does "issue driving licences", but just is "responsible for maintaining ... a database of vehicles "

License plate as what they call a number plate in the United States and Canada. But I’ve heard British people call it a license plate in movies and shows.

Correct, but what I’m saying is that I’ve heard British people call it a license plate as well. American things have a way of catching internationally.

А почему никто до сих пор не предложил исправить НЕ МОГ в переводе на НЕ МОГУ?

Grumbler: Can't STOP, but the reg was a KS something.

✅