Doesn't make sense at all. If your mom's house falls in value, so will the cheaper houses outside of London that she wants to downsize to.
With £320k equity your mom is in a great position overall, just ride this out and see what happens.
On a point of principle - part of the root cause of the housing issue is everyone relying on house price inflation to fund their lives. Its a travesty that we've been sucked into this eternal slavery to debt via the housing market.
Couldn't agree more.
Imagine if your mortgage was half what it was all way through.
Could be mortgage free in less than half the time. And not feel a slave to paying off your debt.