In many of those deals, the lender requires an extra guarantee from the borrower's parents, and the interest ratespayable by the borrower are considerably higher than for someone taking a mortgage with a larger down payment of, say, 20% or 25%.
The rest can usually either be used to buy an annuity (an income for life that will depend on your age, the value of your pension fund, and annuity ratespayable at the time) or to provide an income via what is known as 'drawdown'.