Are we talking actual class (ie, middle class folk earn around £25k), or strange British perception of class (middle class folk earn £50k and send Jemima and Tarquin to private school)?
I always thought it was about level of education, and profession, rather than income. David Beckham is hardly middle class. Doctors, Teachers, Lawyers, etc = Middle Class. Of course some professions are pretty specialist and don't require high level qualifications, yet still command large salaries, but I suppose anything that requires a fair amount of specialist training/study/skill could be considered a Middle Class profession, whereas something that doesn't could be considered Working Class. Progression to high level within an industry could elevate the person to Middle Class status as well; you could in theory progress from shelf-stacker to managerial level, but you would need to possess particular skills and abilities to do so in a very competitive environment.