Between the Middle Ages and the 20th century, the UK economy had gone through periods of growth and periods of decline. During the Middle Ages, a very simple but successful system of trade, both at home and abroad, helped the economy to flourish. There was poverty during the Middle Ages, particularly in some areas of the City of London and just outside of it, but this was not nearly as severe or widespread as that found in Victorian times. Such was the economic prosperity in Britain at the time, that many charities existed to help those less able and less fortunate.
Agricultural systems and the start of mining first came about during the Middle Ages. The Industrial Revolution then saw huge technological development which brought with it both great economic success and pockets of overcrowding and poverty. The increase in population meant that charities were no longer able to successfully deal with this by themselves and the continued involvement of Great Britain in overseas wars knocked the economy at various points. After the World War Two, things generally picked up and the new welfare state helped even the poorest people to find a place in society.
If you wish, you can study the Industrial Revolution in more detail.