Secondly, the loans that are secured this way are generally restricted to a limit of no more than 60% of the value of the home, including the charge or further charge.
Thirdly, the parents income (as guarantors for the loan) is usually taken into account for affordability purposes in the event that the payments fall to them. This is the usual position if the loan applicant defaults on the loan payments when due.
Guess you haven't been reading the papers or watching the telly.
The whole reason we have this here recession is cuz banks were loaning 80, 90, 95, and even 100% of appraised value, and were not paying attention to incomes AT ALL.