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A quick update for anyone who's interested .... after a phone call, I've found out that the annual cost of £25, for putting the plate on retention, was abolished a few years ago.
Your certificate lasts for 10 years, after which you'll need to renew it for free.
It's £80 to assign a number to a car and £80 when you want to take it off; so if you decide to sell your car with your personalised number on, it'll cost you £160 to take it off your current car and assign it to your new car.