[Brutha]

For crying out loud, modern England was founded on the "Original Constitution," the Magna Carta.

The Magna Carta certainly does nothing to prevent the parlament in the UK today to take freedoms away from citizins. But their are a lot of other things in writting like the human rights (those have also be signed by the US by the way), that provide an understanding that the parlament isn't allowed to do certain things.

Eh? First, Britain isn't exactly a shining beacon to the free world.

The US isn't either with nice little laws like the patriotic act.
Your constitution certainly didn't prevent that patriotic act.

from wikipedia: Because the UK has no single codified documentary constitution along the lines of the Constitution of the United States, it is often said that the country has an "unwritten constitution".

Sure, they are laws that describe the powers of the parlament, but those don't have a special status of being unchangeable.
They are normal laws, that change and evolve over time.

Values are subjective and language is inefficient. One man's idea of "freedom" is different than another man's. That's why it is necessary to define these values in written word.

Your Constitution doesn't do that. When it is written it was for example understood in way, that didn't gave black people the same rights as whites.
Things like the right to the freedom to have sex with your adult partner the way you want would now be understood as being granted as basic rights by the constitution.
When the basic understanding of freedom changes and a majority (as in a majority that controls the public forum and the three branches of goverment) sees the concept of freedom different than you, citing the constitution won't help you.

Do you hold this same line of thinking for other agreements that typically require a contract?

Most

If you made a business transaction with someone, would you simply accept their word for it instead, because they claim to have the same values as you?

When Nafta would transform into a souvereign entity you can be quite sure that there will be a massive amount of paper involved.

But contracts change and have to be kept up to date.

In addition a contract that I make with another person gets it power through the laws of the state in which I live (I can sue on legal grounds my relativly powerless partner if he doesn't uphold his part).
I can't do the same with a state, when that state acts uniform (the three powers agree on what they want to do). You need a powerful third party to uphold written contracts.

Apples and oranges. The United States went into Iraq and imposed a new government on them. They didn't bring it about themselves. I agree that it doesn't seem like the people of Iraq want a republic, but that doesn't imply anything about the necessity of a written constitution.

It an example of what a country with written constitiution but without a "cultural constitution" looks like.
It a extreme.