Owning your own home

The idea is simply that the amount OVER AND ABOVE INTEREST that he paid into the mortgage (the principal repayments) - could have been invested elsewhere.

After 20 years he has a £22,000 nest egg in an ISA.
He uses THAT to pay off the mortgage.

Ah, I see what you are saying.

But it still doesn't seem right to me, on interest only you'll be paying the same rate of interest over the life of the mortgate (assuming rates don't change).

With repayment you will be paying progressively less interest, so the interest you gain on the ISA is cancelled out. As the banks have to make money the rate of interest they charge you is always going to be more than they pay you.

It's a interesting idea, but I wouldn't do it.