I think it's because people have the impression that the class system is determined solely by how much money or 'stuff' you have - and a lot of working class people have acquired wealth in the last few years - by things such as winning the lottery, buying their council houses cheap and then selling them for big bucks etc.
French sociologist Bourdieu suggested this too - that in the late 20th century and now we are judged by others on our cultual capital more than where we were born or what our parents did. What we do for recreation (the holidays we can afford, luxuries etc) is more a statement of ourselves than what we do for a living. Thus the builder/property developer is self employed but has a big house with a pool and some minor celebrity friends.
Anyway I promised I would post the Labour model published in the Guardian in 1999 outlining the new class structure for the UK.
Class 1.1 Higher managerial and professional occupations: Company directors, Corporate Managers, Police Inspectors, |Bank managers. Senior civil servants, military officers.
Class 1.2 Higher professionals: Doctors, Barristers, Solicitors, Clergy, Librarians, social workers, teachers.
Class 2 Lower managerial and professional occupations: Nurses, midwives, jouralists, actors, musicians, prison officers, police , soldiers
Class 3 Intermediate occupations: Clerks, secretaries, driving instructors, computer operators, telephone fitters.
Class 4 Small employers and own account workers: Publicans, Playgroup leaders, farmers, taxi drivers, window cleaners, Painters, Decorators
Class 5 Lower supervisor craft and related occupations: Printers, Plumbers, butchers, bus inspectors, tv engineers, train drivers.
Class 6 Semi-routine occupations: shop assistants, traffic wardens, cooks, bus drivers, hairdressers, postal workers.
Class 7 Routine occupations: Waiters, road sweepers, cleanersm couriers, builsing labourers, residue collectors
Class 8 Never worked/long term unemployed.
The interesting thing is that although this was published in 99 it is already largely outmoded. Life style choice is superceding occupation as the guiding factor to how people live. I know a few typcially white collar workers as directors or high paid managers choosing to switch to skilled hands on jobs such as plumber and electrician because there is a huge demand in the UK for these services delivered on a quality basis so it pays extremely well but also allows much more time at home with the kids and family. I also know a number of very well educated couriers who could be computer operators but choose this high risk job for the monetary benefits and flexible time which allows them better holidays / leisure pursuits.