What it means is that 65% of homes are occupied by their owners. It doesn't mean that 65% of people live in their own homes, but I suspect it is not too far off the share of people living in their own home or in a home owned by a family member or partner.
I guess the difference is people renting a room in a house occupied by its owner. Did I forget anyone?
It may not be the same at all in big cities though. I know that homeownership rates in London have collapsed far below 50% in recent years.
https://fred.stlouisfed.org/series/RHORUSQ156N
https://tradingeconomics.com/united-kingdom/home-ownership-r...
https://www.ethnicity-facts-figures.service.gov.uk/housing/o...
Theres also the question of what it means to own ones house. A lot of people have a mortgage for their house, but the relationship between this and benefitting from the value appreciating is different for different people.
"Households in London were fairly evenly distributed across the tenures, with half in owner occupation and half in the rented sectors."
Section 1.10 in https://www.gov.uk/government/statistics/english-housing-sur...
To get the number of voters you could weight by average occupancies but I don't know what those numbers are.