Rich, is your present plate a personalised number?
If not, and it is the number which was originally issued to your car, the number is held by the DVLA until such time as you sell the car, when it goes back on. It is never used on any other car, so hang on to your number plates as they could go back on if you sell the car sometime.
If your present plate is a personalised number, then as someone else said, it would have to be put on to a retention certificate at a cost of £25 per year.