Just to clarify, this suggests that your parents earn total around 16000 a year, pre-tax [0]. Perhaps your guess was accurate, and they really do - but I also find that the average person is normally wildly off in their estimates of the UK wealth distribution. A few thousands more a year would already place them in the top 50%.
[0]: https://www.gov.uk/government/statistics/percentile-points-f...
My parents are seperated, both lived in social housing at that time.
In 2022 they earn approximately 16k between them. Think minimum wage 20 hours a week, that's the ballpark.
I looked at the table of individual taxpayers income. For the relevant years, my mother's income was actually probably closer to the bottom 10% :)
There is a skew here in that you need to pay tax to be included in the table at all, though, hence 2018-19 having 1% set at 12,100 because that's the income tax threshold.
The percentage doesn't really matter though, basically what I'm saying is that like, it's useful to have a place to dump your belongings. That can be a parent, friend, whatever. It's a social capital thing more than it is a monetary thing unless you own tons of stuff and are unwilling to part with it.