Whilst it is true that multi day power failures have occurred previously without leading to widespread disaster, I feel that an organised attack affecting a whole nation would be much more serious.

In previous breakdowns, food has still been available, perhaps from smaller owner operated shops that remain open without power, or from larger stores with standby generators.
In a national event, such supplies would soon be exhausted and replenishment not possible without electricity at the wholesalers premises.

In the UK I expect considerable disorder from those who have not received social security payments.
How are these payments to be made without power at the government offices ?
And even if the payments are still made, how are the claimants to access the money, with all ATMs and banks shut.