There are definitely a million shades of grey in between and indeed the lighter ones are more probable.
That said, the ultimate attack would not be drones, missiles, bombs, invading hordes, zombies and whatnot. The ultimate attack on our society would be switching off power and communications. Not hard to achieve, and we seem to be volunteering for the loss of power in the years ahead anyway.
That would have the country thoroughly shut down in hours, and create severe repercussions lasting far longer than 72h, on every level imaginable. Given the level of preparation of the majority of people, it would be a few days at most before it started to go South very quickly, and no amount of personal preparation, compassion, or anything in that matter would be able to restore things to a resemblance to what it used to be; we live in a 'just in time' society with massively long and overly complicated supply chains, and most things we do rely on some sort of connectivity and access to power.
From very personal experience, I know that -no matter how prepped one is- they will just prolong their misery for a little longer if something like that happened; we will eventually lack not only the material supplies, but have also lost a lot of the 'software' in our head to know what to do past a certain point.
The government should -instead of finger-pointing and telling people to prepare for war- prepare itself and the country, by ensuring society would not collapse overnight if a massive cyber attack happened. For that, it would take a lot of attitude changes, creation of backup systems that get tested regularly and are known to work, and keeping a lot of things in reserve that might never be used; something the UK is unlikely to ever spend any money on given it has recently been cutting down on everything -including essential armed forces training.
Other than that, the general public having a few days worth of supplies is a good idea. To be honest, it baffles me to think that there may be people who don't have enough stock at their place for 3 days. Surely, one keeps more food and basic supplies at home than that!
+1 on the foreign section of supermarkets. You can get some real gems there!