Just make contact with the companies selling plates. They used to offer you a price to buy outright off you, and a commision based sale for them to sell it on your behalf. Sadly they will offer you a lot less than they sell them for obviously.
I would suggest looking at similar plates to what you have and look at the price they are asking. Then price yours around that price.
At the end of the day it is worth only what someone is willing to pay for it. None of my dateless plates (pre 1963) have cost me more than £95 to buy the car they where attached to.
First plate attched to mk3 Cortina, cost £150, £80 transfer fee, sold car for £200.
Second one attached to Morris Minor, car cost £95 transfer £80 car sold for scrap, scrap price eleudes me at the moment.
Third plate attached to Hillman Minx, car cost £395 transfer fee £80 car sold for £400.