I'm not sure people are going to be able to argue that they didn't understand that the words 'interest only' meant exactly that, unless they are also going to drop themselves in it in not being honest about all the declarations in the mortgage paperwork they signed which are likely to have included statements like 'I understand the repayments will not repay the capital borrowed' and 'I have other investments or arrangements to repay'.
Or will the basis of the complaint be that the banks should have protected people from themselves in this regard?
lucky You are not being stupid, but your example numbers may not reflect the reality. Eg, I used a mortgage calculator and a realistic mortgage amount and interest rate to come up with my figures, where yours look like they were plucked out of the air
but will be correct for some combination of mortgage term, amount and interest rate, but I CBA working out if it's a realistic one.
Scaling my figures up, if the repayment on a repayment basis was £1000 pm, the interest only payment would be about £530 a month, so quite a big gap. Also, depending on finances, even an extra £200 pm might make the difference between being affordable/comfortable and not.