some people are genuinely poor/unlucky then again we're all just human and prone to making mistakes too
there are a lot of issues with innumeracy though in the UK and just general lack of planning - especially when you're conditioned into being used to the idea that when you hit rock bottom the state is always there to give you additional help.
Fact is wonga is a successful company, chavvy areas have electrical stores offering goods on really really bad value schemes whereby the chav paying for a new TV ends up buying it on credit, at full RRP, and paying several times over (a bit of patience and they could have had a few hundred in savings instead). They total price isn't the figure emphasis is put on but rather the weekly payment amount.
And it isn't just the underclass - working class and middle class people seem to be quite adept at blowing money they don't have. A fair few people who were in the same grad scheme as me bought BMWs and Mercedes within the first year of starting... granted some of them lived with parents and weren't paying rather expensive London rents but still, straight out of uni with a load of debt and they think blowing 5 figures on a car (on credit of course) is a good idea.
I reckon that while there will be a small % who have been massively unlucky and ended up in debt through no fault of their own there will be many more from various walks of life who simply haven't bothered to plan for the future.
One person I know is reluctant to get married to his current girlfriend as, though he owns his own flat, she currently has £XX,XXX in credit card debt and loans... and has done for a few years now despite being in full time employment, in a good job and in a position to stop spending at the current rate and pay it off.