Do a Google search.
You could try a coin dealer.
You could try eBay or a reputable coin dealer in your area.
Bring it to a reputable coin dealer and they will identify it and give you an opinion on its authenticity. Most coin dealers will not charge you for this service.
A reputable coin dealer will be able to value your coin. If the coin has been circulated, you might be lucky to get 50 pence for it.
You could take it to a reputable coin dealer or advertise it on eBay.
There is no documented evidence of a minting flaw in the "Festival of Britain" Crown, but it is possible. A reputable coin dealer will be able to advise.
You could try a reputable coin dealer in your area or eBay.
You could try a reputable coin dealer in your area or eBay.
Your best bet would be a reputable coin dealer. Alternatively, you could try eBay.
Unless the coin is mint uncirculated or Proof, you are not likely to get much for it. You could try a reputable coin dealer in your area.
These coins do not have a minted value and do not appear in coin catalogues, but they are collectible. The only helpful suggestion I can make is to take it along to a local reputable coin dealer for valuation.