I can't really spend a lot of time on this right now to figure out why, but your data seems to be in direct conflict with a sister comments figure about home ownership in london. I don't want to draw any conclusions for which figure is correct, perhaps you have more domain knowledge and can explain?
None of the links in the sister support home ownership being below 50% in London. The third link shows a difference between white and non-white British ownership levels but that's it.
"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.