Explain why there was widespread unemployment in 1815

One of, if not the main factor when considering British unemployment in 1815 is the end of the Napoleonic Wars. The Napoleonic Wars proper, began in 1803. From 1803 to 1815, the United Kingdom was essentially at 'Total War'. Whilst Britain, relatively speaking, was not at a great level of comparative economic loss, compared to other European powers, it is undeniable that the wars had a fiercely negative effect on the British economy. The British national debt had soared, with the cost of servicing the national debt reaching more than 50% of GDP. British private investment was heavily focussed on supporting the government and British spending had been focussed solely on the war, not on the infrastructure that had been powering the Industrial Revolution. 

The state of the British state finances was not the only wholly negative aspect of the Napoleonic Wars on the British economy. The workforce was thoroughly unsuited to a peactime economy; from 1793 to 1801, the number of British sailors had increased by a factor of nearly 10. The 750,000 British soldiers and sailors who fought for Britain were almost all trained in nothing other than warfare, something that wasn't quite as useful come peacetime, the same can be said for many of those who manufactured the weapons of war. This resulted in the alleyways of London, as well as most other British cities, becoming filled with traumatised, alcoholic, violent ex-soldiers. 

There were undoubtedly other factors leading to unemployment, such as the lag in supply meeting demand, caused by an indynamic economy. Taxes had risen to service the national debt, stifling entrepreneurialism.