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These new reg plates can only be assigned to brand new cars registered on or after March 2021, and that means there will be several months before these vehicles even become available (the new number plates become available in advance of the vehicles), so the plates will first be supplied on certificates with an initial lifespan of 10 years.
This means you can secure your ideal new number plate now and you have a full 10 years before you need to assign it to a vehicle, or even renew the certificate further. That's plenty of time to obtain a suitable vehicle, but you don't even need to own a car to buy a new car number plate. Many buyers secure plates purely as an investment, never assigning them to cars and looking to sell their purchases later on.
If, however, you are lucky enough to be purchasing a brand new car when the 21 new registration plate cars hit the road in March, then you can register your new number plate straight to the vehicle when you pick it up!