If all the cars were electric, you'd have on average 20 millions * 50kWH = 1,000GWH storage capacity available to put back to the grid.
If you look at the cost of griod level storage batteries, it's about $200/kwh or $200 millions per GWH.
Yeah and everyone is going to be really pleased to have the grid power cycling their VERY expensive battery, and shortening its useful life as a car battery, aren't they, just so Miliband can grin his stupid head off and claim he has abolished carbon emissions from the power grid. But the idea doesn't work anyway. High pressure systems can last for several days at a time paralysing wind generation, and under your crazy idea, flattening everyone's car battery so the country literally grinds to a halt. Great idea..... I can just imagine the government announcements - covid style with Miliband grinning, standing at that Covid lectern, and announcing, 'STAY AT HOME'. This would be happening several times each winter, and in other seasons too - like right now and yesterday.
Right now we have an installed wind farm capacity of 30 Gigawatts and the entire output is at this moment 1.1 Gigawatts..... We can't build our way out of this problem of intermittency by making wind farms. I'm not against wind - on the contrary, I think it is great in general times.
What we need is REALISM about use of gas for times like today. Miliband is a lunatic.
DANGEROUSLY MAD
EDIT:
By the way - your numbers don't add up. That 1000 Ghwhrs of car battery capacity you conjecture, would if all the batteries were totally depleted, amount to 28 hours of winter grid use.
In practice, you could not drain them that far, so where does that leave us - even if people would allow the government to crash into their car batteries and leave them all stuck with a flat battery at home. Then - what would happen after the calm, when all those batteries were being simultaneously charged from flat? Grid demand would be absolutely massive.
People WON't TOLERATE that kind of government assault on their lives and neither should they.
I think they will just drive onto motorways and city centres and switch off and wait there until governments fall. A few half witted ar seholes setting fire to bins and throwing bits of wood is not the only kind of mass civil disobedience. This country is turning into a paradise for fools and overbearing government.