As others have said it's not so much about how much you earn but more about where you have come from, your culture capital and your views and beliefs.
Upper class - Royal family, aristocracy, people with titles and land that have been in their family for centuries. Anyone with an officially registered family crest. Think opera, ballet, theatre and the Arts.
Middle class - families with history in the professions or politics, think doctors, judges, barristers and solicitors, MPs, civil servants etc. Children are privately educated.
Skilled working class - plumbers, electricians, mechanics, gas technicians, joiners, operators of complex heavy machinery (cranes, trains, trams), locksmiths, carpet fitters, painters and decorators etc. Some children may make it to grammar school.
Working class - minimum wage jobs working in retail or customer services, labourers, cleaners, bin men, postman, delivery driver, truck, bus driver, taxi driver etc. State educated.
Underclass - benefit cheats, never worked a day in their lives and neither have their parents or their 50 year old grand parents, history of criminal convictions and substance abuse in the family. Fond of cigarettes, booze and gambling. Somehow manages to have the biggest TV and the latest IPhone.