Added to that, when your fixed rate ends, if you're in negative equity, no other mortgage lender will lend to you, so you are stuck on your mortgage lender's SVR. Let's do some sums on that.
Let's say I bought at 465k house in May last year when I was offered a 2.77% 2 year fixed rate:
25 years
2041 pcm
(5% deposit, 23,500)
Now, let's say I took a 2 year fixed rate term and am in negative equity come May 2024:
23 years
403,179 outstanding
3199 pcm
That's on an average SVR of about 8%, per current rates:
https://www.landc.co.uk/mortgages/svr-watch/
But, we're actually expecting interest rates to rise by another 1% this year. Let's assume lenders try to absorb some of that and increase SVRs by 0.5%, taking us to an SVR rate of 8.5%:
23 years
403,179 outstanding
3330 pcm
That is a 63% increase in monthly mortgage payments, for which you get no choice, unless you've put yourself in a financial position whereby you have been massively overpaying to avoid negative equity.
So, yeah. Negative equity absolutely matters.