Rare coin in wrong metal to fetch £2.5k

There are thousands of old pennies kept by nostalgic collectors in the UK. Most are worth £1 at the very most - even if they are in brilliant condition. The damaged ones are worth their weight in scrap. However, one is about to go on sale, and is expected to fetch at least £2,500.

So why is it worth so much, and how does it compare to the most expensive coins?
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