It's not bad altogether, but the devil is in the details. :wink:
Let's look at the first paragraph.
The first sentence doesn't need the definite article (the). Also, use at instead of from. The third sentence needs to be entirely rewritten. (Say: without being accused or convicted of anything.) Tell the reader what CAMBIO 16 is."Andres Ortiz is the prisoner number KW1769 from Feltham jail, on the outskirts of London. He is behind bars for being an illegal immigrant. Without being accused, courted or a convicted prisoner, he pines away since 16 months ago in a prison for juvenile delinquents, surrounded by people who has raped, killed and robbed. There he received CAMBIO 16.
Let's look at the second paragraph.
In the first sentence, what you want to say is that he doesn't want to admit he is from Colombia because he doesn't want to be sent back (deported). The second sentence is a perfect example of when the past perfect should be used. Rewrite the third sentence, and make it two sentences. You don't need the comma in the third sentence. Thus, it should be: "Then the threats started." Say a half dozen or half a dozen. Say the stall was burned down or burnt to the ground.His story starts in a Colombian town, which he does not want to mention because of the risk he takes of being deported. His stepfather, Roberto Gomez, bought a stall to sell hot dogs at the beginning of 1987 without knowing that, the former owner also used it to sell drugs. When the local mafia required him to continue with the business, he refused in several occasions. Then, the threats started. He received almost a half a dozen of anonymous letters. In the end, the stall was burnt and he was required to leave the country.