Is An Interest-only Mortgage A Good Idea?

It depends what happens to the housing market long-term.

It may let you buy a house now, if you are convinced that you will never get on the property ladder otherwise (with how house prices are rising relative to salaries).  If prices do keep going up sharply, then just buying a house and paying only interest on the mortage will still get you quids in (eventually).

However the whole point of buying rather than renting is usually to save money in the long term (money towards rent could be spent buying equity in your house, instead).  And house prices may not go up that much more.  As long as you have no plans for paying off the debt, you could sell your house in so many years and be worse off than renting, because you had to pay all the house-buying fees (soliciter, stampduty, estate agent, etc.) as well as what you probably would have paid for renting.

And if house prices fall... well, you could end up much poorer over renting, unless you have acquired lots of equity in your house by paying off at least some of the mortgage.