Overview
In 1971, the United Kingdom officially moved from the centuries-old, traditional Imperial system of currency to a new, decimal-based one. It was the end of an era for some; no longer would one receive Victorian pennies with his change, no longer would such concepts as 'shillings' and 'half crowns' be such a part of the nation's identity.
But as time has gone on, we have at least accepted this new-fangled money, and even as more and more of our traditions are destroyed in the name of progress, the majority of the country appear to love the system enough to openly resist the Euro, much to the annoyance of Mssrs. Blair et al.
So, for those not yet acquainted with that wondrous thing that is the British Monetary System, here is a guide to what one can expect to find once he reaches the UK.
British Decimal Currency - 1971 to the present day
There are 100 New Pennies to the British Pound Sterling. One pound is written as £1.00, while one penny is simply 1p. A pound and a penny is written as £1.01 - when using both pounds and pennies, the p suffix is removed. Therefore, one would never write £1.01p.
Paper Money: (value, colour)
Fifty Pound Note (£50), pink
Twenty Pound Note (£20), purple
Ten Pound Note (£10), brown
Five Pound Note (£5), blue
One Pound Note (£1), green
The £1 note was replaced by the £1 coin in 1983, and ceased to be legal tender on March 11, 1988. Scotland still produces £1 notes, however, which can be spent in England provided that the retailer accepts them; this is not as likely as one would think.
All banknotes issued by the Bank of England are a different size and colour depending upon their denomination. the £50 note, being worth the most, is the largest, with the old £1 being the smallest. (This honour is now conferred upon the £5 note.) This makes identifying notes at a glance significantly easier than with American banknotes, where all are the same size (and, with the exception of new $20 bills, the same colour).
Moving onto coinage, not all British coins are the same shape. As one can imagine, they are of various sizes, but interestingly not all of them are circular. The Fifty and Twenty pence pieces are both heptagons; all other coins are circular.
Each coin bears a different design on the 'tails' side, while upon the face is the Queen's head. Upon the succession of the Prince of Wales, his face shall replace Elizabeth's, but all coins bearing her face shall remain legal tender. Traditionally, every time a new monarch's coins are minted, their head is facing the opposite direction to their forebear's.
Coinage: (value, design)
Two Pound Coin (£2), "Four Ages"
One Pound Coin (£1), Varies
Fifty Pence Piece (50p), Britannia
Twenty Pence Piece (20p), Tudor rose
Ten Pence Piece (10p), Lion
Five Pence Piece (5p), Thistle
Two Pence Piece ("Tuppence") (2p), Crest of the Prince of Wales
One Penny (1p), Portcullis
Half Penny ("Ha'penny") (½p), Crown
The £2 coin was only introduced in 1997, whilst the halfpenny was removed from circulation in 1984. Sporadic issues of £2 coins were made before '97, usually as part of special sets of coins, but the design was different to the current and these older coins are of different dimensions. The 20p coin was introduced in 1982, and is considered to be a very useful little coin indeed.
The design of the £1 coin was originally the Royal Arms. This is changed every year, with a design only having been repeated three times. The £2 and 50p coins have also carried alternative, commemerative designs upon them at different points.
As five pence was the decimal equivilant of one pre-decimal shilling, the 5p and 10p coins were initially minted as the same size as the one and two shilling coins. They were interchangable during the period when decimal currency was being introduced, but eventually the fivepence was reduced in size, and the thickness of the tenpence reduced. (lj informs me that the new tenpence and old fivepence are the same size.) The 50p coin has also been reduced in thickness. All others remain the same as when first issued.