Hhhmm. If the MIL wanted to make a £50,000 gift, specifically to pay off some mortgage debt, then this is a kind and generous offer that on the face of it, most people would snap your hand off for. But the issue here is that the gift is being offered with the proviso that you match her contribution.
My worry here is that the gift isn't really a gift, its a hook to maintain control of your finances and limit your ability to make your own financial decisions, which as an adult with capacity, is your absolute right.
If it were me, I would spell out to her, that you are extremely grateful and such a gift would have a huge beneficial impact, but you would be in a more robust and nimble financial position by retaining £50,000 as an emergency fund while using her generous gift to reduce the mortgage debt to the point where interest is minimal, allowing the principle to be repaid a lot faster. Her responses, I think, will offer a window into her true motivations.
It comes down to what degree you are willing to hurt yourself financially to maintain your independence and agency. Everyone has a different red line on this, but there is a point where you say "thanks, but no thanks" and go your own way safe and secure in the knowledge that every penny you have is off your own sweat.