It isn't actually a coin: it's a medal. It was struck for the British Empire Exhibition in 1924, so is 99 years old. It was more like 49 years old when someone used it "accidentally" to pay for their gas.
You can buy one of these on eBay right now for £25.68, which sounds impressive but you can also buy one for £0.79 in the same condition.
I don't think I'll be trying to sell mine if the stamp I put on the envelope is worth more than what's inside it.