People moving from higher value areas to lower value areas (shitloads of Londoners have made shitloads of money moving out*). People downsizing their properties when their kids leave home (you might easily get £200k cash out of it in many areas, moving say from a 500k family house into a 300k retirement house). People remortgaging their properties to subsidise expansion of the property, or doing equity release to fund consumption. Also people do of course sell up and go to live abroad, not just at retirement.
*To give an example I know something about, the property my co-op bought in London a couple of years back cost £500k. It was bought from a working class family who had bought it in the mid 2000s for £120,000. They were moving out to a cheaper location on the edge of London where they could now buy without a mortgage and have cash left over.
PS I'm trying not to exaggerate the scale of winnings here. In fact a fairly normal family home with 4 bedrooms round me now costs more like £700k but £500k is 'normal' in a lot of the south east and other expensive urban and rural pockets around the country.