You've not mentioned the date when the overdraft was granted. It's only very recently that banks have been required to consider customers outgoings, as well as their incomes, when deciding whether to permit any form of loan.
Someone living in their parents' home, for free or paying only a nominal amount, and earning £18k pa could easily service a debt of £5k (even at the interest rates charged by credit card companies, rather than the preferential rates given to overdrafts) whereas someone earning the same amount but renting a flat on their own in London (if that's actually possible!) wouldn't be able to do so.
So an income of 'only' £18k (which, incidentally, is more than double mine!) wouldn't be an automatic barrier to a £5k overdraft and thus, until lenders were required to examine customers' outgoings, would not constitute 'mis-selling'.