I have read elsewhere that some numbers are indeed recycled - especially highly desirable pre 1963 numbers where a) it can be shown the car no longer exists and has not existed for a very long time , and b) where DVLA can sell a highly desirable number for a lot of money .Totally not true, the number previously allocated to a car which then has a cherished number put on it is reallocated to that car when the number is moved off. I keep a stack of number plates in my garage so I can put the old ones back on where the car had a previous plate assigned
what I would accept is that if a number is no longer in use, car scrapped/exported then this might be true, but even then there are exceptions such as proving historical use of a number if a car has been reimported
Where it could fall down is if a long lost ‘barn find’ surfaces with a number that has been sold on .
However , registration marks remain the property of DVLA and you only buy the right to display them .