Conversation so far:
what is a registration mark on a car
Vehicle registration marks are used to identify vehicles and their registered owners.
where is the registration mark on a car
Vehicle registration plates are the mandatory alphanumeric plates used to display the registration mark of a vehicle, and have existed in the United Kingdom since 1904.

Question: what do the first two letters mean
Self-contained rewrite: what do the first two letters mean on a uk vehicle registration plate