This is not a good way to explain encryption to people. Explaining the concept of a "key" is essential to explain why this doesn't work.

- In your example, the contents can be deduced from the "encrypted" data, without the key. Indeed, there is no key, but rather a complex dictionary transformation.

- A "backdoor" is merely teaching GCHQ to speak English. Sounds perfectly reasonable in your example.

I'd argue the exact problem is that politicians have the particular understanding of encryption that you just gave.

You need to communicate two things:

1) Why backdooring safe encryption irreversibly breaks it for everybody

2) Why that's a bad thing

Guys, both of you are behaving like non-tech people are idiots. They are as smart as you. They often have college degrees. Even children understand perfectly well what encryption is. Boy/girl scouts go out of their way to teach it, etc.