The Kew Garden 50p could be worth £20

The Royal Mint has revealed the UK's rarest coin in general circulation: the Kew Gardens 50p, released in 2009.

The coin was released to celebrate the 250th anniversary of the Royal Botanic Gardens - but only 210,000 ever made it into circulation. While this might seem like a lot, it's a tiny number compared with, say, the 22.7 million 50 pence pieces doing the rounds bearing the 2008 shield of the Royal Arms design.

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