How do people get those number plates like this: XXX 1234?
Sounds like you’re talking about Northern Ireland plates, which are often sold in the UK as cheapo personal plates. Beware if buying a car with those plates that hasn’t come directly from NI though, a lot of dodgy dealers will put a £99 NI plate on a car when they want to make it harder for you to check the identity. ie the car could be stolen or badly accident damaged.
