You can take equity release, but pay the interest, so that it does not roll-up at compound interest.
We investigated that, but were excluded because, for some reason you cannot get equity release if the footprint of any flat roofed extension exceeds the 35% of the footprint of the whole house. So we took out a lifetime mortgage instead. The difference is that with the equity release we could have stopped paying interest at any time or for short period of time, if we wished, while with the lifetime mortgage, if we miss any interest payment we are in arrears with all the consequences that could follow.
I am puzzled that you managed to get such a large interest only loan maturing after you had reached retirement age without needing to provide evidence of how you meant to repay the capital when the due date arrived. Or was the house itself the security for the loan and it was agreed that if, you could not repay the loan from other sources, the house would be sold to pay off the loan.
Either way your DH is giving a good impression of an ostrich, with his head in the sand, hoping that if he ignores this problem, it will disappear. In fact if he continues to act as he is now he will be devoured. The mortgage company will repossess your home and sell it cheaply in order to effect a quick sale to get their money back and you will have much less than £500k to spend on a new home.