That seems reasonable considering what happened with griddy in texas.
Then if the prices go sky high, users who chose that plan at least get to use 5kWh each day. 5kWh is plenty to not freeze and to keep your phone charged and fridge going. If you want more, you can pay more.
The UK already has a system of electricity meters that have a 'balance', and when the balance reaches zero it cuts off till more credit is added. Such a scheme integrates well with that, and means a user can never rack up a massive unexpected bill if the price goes crazy high or low.
I suspect 5 kWh/day is way to little to keep the house warm and still be able to cook food. The exact number doesn't really matter here. What's more problematic is how the 5 kWh of guaranteed/insured/subsidized power would be allocated.
If it's your first 5 kWh/day, it will be you'll blow past the quota by noon, and if there was a price spike in the evening you would pay the full price of it, or shut off your breaker until prices go down.
Having it as a some sort of pool you can draw from anytime you want doesn't suffer from the above problem, but would make such a service very expensive to run for the counterparty due to adverse selection. Anyone using such a service would opt to use the market price when electricity is plentiful/cheap, and switch to fixed price when electricity is scarce/expensive. The net result is that the 5 kWh ends up always being the most expensive electricity for the day, and a service providing such a guarantee will be priced accordingly. In a competitive market, you would expect the price of a fixed price plan to be the same as the "5 kWh per day at fixed price and anything extra at instant market price" plan that you propose. You might be tempted to argue that utilities/the government can subsidize the latter plan as some sort of green energy initiative, but that would be be bad. The most expensive energy also tends to be the dirtiest, because peaker plants (power plants that can quickly turn on/off) tend to run on fossil fuels. Subsidizing such a scheme would also be subsidizing electricity consumption during peak hours, and therefore subsidizing fossil fuel plants.
Then you can hear the beep and either decide to turn off a few things, top up your balance, hit the 'emergency 1 kWh' button, or just sit in the dark for 15 minutes.
This benefits the grid operator because they get to massively overcharge for curtailed energy. It benefits the EV manufacturer because they get to greenwash. And the consumer doesn't get a say because they don't actually own their DRM'd remote controlled car and charger.
Yet the vast majority of EV's don't have the ability to discharge into the grid to earn money... Most can't even charge when power is negatively priced unless you pay for a third party charger and service.
Basically, Car owners, charger owners, Utility companies, and EV manufacturers would all like to earn the money by charging/discharging your EV at times to earn the most money. But all of those people don't want someone else being able to take that sweet sweet revenue. So they put roadblocks in the way.
EV's usually have the capability to charge into the grid, but firmware that won't allow it.
Chargers (AC and DC) won't allow power to flow back into the grid, and will disconnect any car who tries.
Utilities refuse to let home users take part in the minute by minute pricing - instead the best available is dynamic pricing set half hourly a day in advance.
And users want to do this, but have three parties who've assembled roadblocks in the way. And there is no indication of that impasse ending anytime soon.