I do understand that, and obviously selling in a slow market is much harder. But if the alternative is eventually having a mortgage you genuinely can’t afford, I don’t think “the market is bad” can really be the end of the conversation.
There are still things people can do.
The first is price. Houses generally do sell if they’re priced at a level the current market will support. That might mean accepting less than you hoped for, which is horrible, but taking a £20k or £30k hit on a sale could still be far better financially than spending years trying to service a mortgage that is hundreds or even thousands of pounds a month more than you can comfortably afford.
And downsizing doesn’t necessarily mean finding the perfect smaller house and completing both transactions at exactly the same time. You could sell, break the chain, rent for a while and buy again when the market settles. You could move area. You could buy something that needs work. There are compromises between “stay exactly where you are” and “lose the house”.
If things became really tight, you could also speak to the lender early. Temporarily going interest-only could work which could buy time while you sell. I wouldn't go interest only long term as I agree that would be kicking the can down the road, but as a short term interim measure whilst you sort out your situation would help.
I think that’s the part I disagree with in some of these replies. People seem to be treating selling as impossible because the market is difficult, when actually it may just mean accepting a lower price or a less convenient move.
If I knew that at the end of my fix my mortgage could become genuinely unaffordable, I personally would much rather make those decisions while I was still in control — even if that meant taking a disappointing price — than wait until I was in arrears and had far fewer choices.
A bad housing market makes downsizing more difficult. It doesn’t necessarily make it impossible.