It all has to be paid back in the end. I could only afford interest only when I took mine out (and yes, I did have endowments to (supposedly) cover, but I now have the situation where time is running out, I have a shortfall, and I will need to do something to avoid effectively renting this property and having it taken off me.
We started off with a part interest (endowment funded) part repayment, but we have recently changed to all repayment because we have a shortfall on the endowment. The monthly payments are now less than they were!