What’s fair

Retireinten

How much equity do you have in your home? Is it not a case of him lending and giving you half of that. In return if gets added to the deeds.

How you deal with the debt is down to you both individually.

On the flip side, if he waits it out then he'd be entitled to a good chunk of equity after a reasonable length of marriage without paying you anything 

TBagpuss

How much non-mortgage debt do you have?
How much debt does he have?
How much is the house worth?

He can't take out a mortgage in his sole name, any mortgage would need to be in joint names, although you and he could have a separate post-nuptial agreement that sets out what you each pay and how much you will each be entitled to.

Is your assumption that if you remortgage he would then pay more than half of the new mortgage, so he was paying the repayments on the extra £40,000 borrowed? 

Is the plan that the borrowing will be short term and he will pay off £40,000 if/when he gets an inheritance?