You pay the £80 and that enables you to put it on whichever car you decide to by using the retention cert. Sometimes its worth letting the dealership register the car and fit number plates for whatever number is assigned, and then do the swap to personal reg once you have the car.
This means you have the original number plates to refit when you change the car in the future, BUT whether you do that will depend on whether you are up for doing the change yourself (very easy), swap the number plates over and whether you are bothered about having the original plates to refit in the future (I always do things this way but yes for some it will be a task)
When you change your car in the future yes you will have to pay a further single £80 (or whatever amount it is) to keep /move the personal registration number