What is the most valuable British coin today? England’s first large gold coin, the Edward III 1344 Double Leopard. The coin sold on 26 June 2006 for a staggering £460,000 (US$841,800) by Spink in London. This is a small amount compare to world's most expensive coin: 1794 Silver Dollar. This coin was discovered and dug up by a metal detectorist in the south of England. Only 3 known specimen for this coin. The two other examples, found in the bed of the river Tyne in 1857, are now both in the British Museum. This is therefore the only example in private hands. It is a slightly different variety to either of those in the British Museum.
The Gold Double Florin, authorised on 14 December 1343, was to circulate at a value of six-shillings. The Double Leopard was an attempt by English king Edward III to produce a gold coinage suitable for use in Europe as well as in England. The florin, based on a French coin and ultimately on coins issued in Florence, Italy, in 1252, was a standard coin widely used internationally, with a value of six shillings. Unfortunately the gold used to strike the coins was overvalued, resulting in the coins being unacceptable to merchants, and the coins were withdrawn after only a few months in circulation, in August 1344, to be melted down to produce the more popular gold Noble.
Edward III (1327-77) gold "Double Florin", also known as Double Leopard, struck in 1344. It had a face value of 6 shillings, and is 35 mm diameter. It was first authorised on 14 December 1343, but was replaced by a new coinage of Nobles on 9th July 1344. It was therefore only issued for seven months.
The obverse of the coin shows the King enthroned beneath a canopy, with two leopards' heads at the sides (the leopard being the heraldic "lion" on the English coat of arms); the legend is EDWR D GRA REX ANGL Z FRANC DNS HIB (Edward by the Grace of God King of England and France Lord of Ireland). The reverse of the coin shows the Royal cross within a quatrefoil, a leopard in each spandrel; the legend is IHC TRANSIENS PER MEDIUM ILLORUM IBAT (But Jesus passing through their midst went his way).
Source: Spink, Wikipedia.