I think they have to be tied to a car, but could be wrong. There may be a way of reserving them, I'm not sure. What I do know is I was pulled over for driving an unregistered car when driving my car with the old number plates, before I had even received the new ones.
Took a few moments for it to click for me as to why they thought my car was unregistered, when I told them the custom plate I had ordered and was waiting to receive, they looked it up and saw it checked out with the car I was driving, but then told me I shouldn't be driving with the old plates..... but I hadn't received the new ones yet (they came a good 2 weeks later than the website promised as well). Was I was expected to not drive the car anymore until I had the new plates? Would be kinda ridiculous cause I need it for work etc...