Even for the people that own their own home there's also the capital required to install insulation, solar panels or a heat pump.
How many of those home owners do you think have tens of thousands of pounds just laying around that they can spend on those upgrades? (and that's after taking in to consideration the grants available)
If you look at the amount of cars driving around in the UK (similarly priced as all the upgrades combined), probably most homeowners. Also, if someone does not have sufficient capital, there are subsidised lease options for all upgrades.
You can buy a car (although not a new car) for significantly less than £10k, so, no, that's a nonsense comparison. A very brief search on autotrader showed me about 80 cars for under £1,000 within a ten mile radius, for example.
Also where the hell can you lease cavity wall insulation from? And what happens if you stop paying, do they come round and suck it back out of the walls?
