Cherished No's can be a very worthwhile investment, I've accumulated quite a few over the years, all have risen in value, just steer clear of the cheap Irish plates! they're awful and your lucky to get any money back!
What is your budget
What are you initials etc
How old is the car that you want the plate to go on
A said above, steer clear of car related plates, unless you manage to find a real corker, which I did recently on 2 occasions!

lateral thinking can sometimes come up with something good
Thom