Why £ for Pounds?

The term "Pound" was originally a weight or mass, as is still used today in Imperial weights and measures. Early on English currency was not tied to the gold standard, but the silver standard, and at this time a pound (in weight) of Silver would cost 240 pence or 20 shillings. Clearly this would make for a cumbersome silver coin, so a gold coin was produced of the same worth, the sovereign. Until England adopted the gold standard, however, the value of the sovereign would fluctuate as the price of gold changed against silver.

Whilst the pound is a very old measure of weight, it wasn't (as far as I know) actually used by the Romans themselves, the Roman language, latin, was used by alchemists, scientists and apothecaries well into the 19th centuary. Apothecaries (medieval chemists/pharmacists basically) decided to adopt the latin word libra in their writing to mean pound, and at the same time defined the pound weight (along with ounces, grains etc), which are to this day called "apothecary pounds". We are still familiar (particularly in the us) with the lb abbreviation of libra for pound weight. The pound sign is simply an L with a bar across, although it is still drawn in an old-fashioned copperplate style.