That was the amazing thing; in most post apocalypse films the saintly heroes are under attack from bands of killers. In this film the heroes ARE the killers!
In fact they were worse than most. The farmer and the village with a road block, get the drop on the Custances and then tell them to move on, unharmed. The Custances kill the farmer and the soldiers at the roadblock, without even trying to disarm them.
John Custance is killing his governments soldiers at the road block just a few hours after he's had to abandon his normal law abiding life, without batting an eye lid.
There was a lot wrong with his tactics. Why, when you know you are going to bug out, be unarmed? You could buy guns in 1970s Britain. Why travel during the day? Why have no backpacks in the cars in case you lose them or they simply break down?
Other things like using a railroad crossing as the perfect ambush (they don't even realise why they are being stopped) and carrying back-up concealed handguns, in case someone gets the drop on you and you, I'd never thought of.
The 'no cars; they are just an invitation to an ambush' made me think too.
A lot was wrong with the film as you say:
Why were his the only children in the film and why with famine in asia, were the rest of the world not planting non-grass crops?
But the film and the book aren't guides to surviving the end of the world. They are about what we'll do to survive.
Imagine the tv announced "the country has 1 weeks supply of food and no way to replace it" NOW. what would you do to ensure your family lived?
The book is a lot better than the film.
Did you notice the boys saying 'global warming is going to melt the icecap' in clip 5? This is a film made in 1970. So much for the 'global warming is a new invention' myth.

> "hope something like this never ever happens!"

It's happening now; there are food riots all over the third world as prices rise......
The Sock