Feedback for an idea

So, it's been a while since I've written anything dystopian, but here is my idea:

It's basically set far in the future after the world has been utterly ravaged by nuclear war. England, one of the only functional countries left, is where this novel is set.
However, things are very different to how they used to be. Though functional, England does not have unlimited resources. They can't support everyone indefinitely.
The essence is this: at twelve years old, people are subjected to lengthy tests that compare all of their aspects, from intelligence to fertility, to estimate how much use they will be in terms of sustaining the nation. From this, they are given a Life Expectancy - an age that the government allows them to live to.
Until they are deemed more trouble than they are worth, and they come for them. And kill them.

Anyway, that's the basic idea. It may or may not need more explanation. I have a whole red carpet of separate character-orientated plot that will serve in the story, but I just want to know what people think of the foundation world that it's set in.

There are a couple of things you might want to consider:

Why is the test conducted when they're 12 and not another age?

Why is the test a one-time thing?

Hunting people down takes resources, so how are those costs justified?

What are the main views of the system, and how do these views impact your characters?

How did the system get put into place, and how is it enforced?

Dystopias are rather difficult to pull off well, and the more thought you put into the background of how the dystopia came to be and how it works, the more plausible it will be for your readers.

I have considered some of these

The test is conducted first when they are twelve, but is reinforced repeatedly throughout their lives every seven to ten years to monitor any change (this it isn't a one-time thing).

I'm still working on the 'killing takes resources' thing, but I'll find some justification eventually. XD

The story's primary focus will be on the resistance surrounding the issue, with my MC being a character who has only one year before her life runs out.

I imagine that I will elaborate on the enforcement and reasoning for the system in the novel.

Thanks for the feedback!

Just a few other things about the context.

Will the city be like the one out of Blade Runner? pathetic fallacy weather and all. or will you try to be unique about it?

How is the city run? what are the police like?

Are there other cities that you can link with to extend the plot?

You may want to plan out the plot and cities and of factors you may think up and want to include.

Yes, I'm not going to start writing this until I've done some serious brainstorming.
Though I don't know what the city in Blade Runner is like, I'm definitely going to try and be unique. Thank ya for the pointers.