In all honesty Elliot, I truly believe that if you really want to do something - then the only thing stopping you are your thoughts!
Let me change your perspective:
Henry Ford, was an uneducated man with very poor "academic" knowledge about his undertakings.
However, in the 1800-1900s he became one of the richest men in designing the T model car. ALTHOUGH his almost nonexistent schooling suggested that he could not surmount to anything in life.
Because of this premonition that a good education equals success, - everyone around him did not believe that he could achieve some of his craziest ideas at the time. (a machine that moves by itself?! impossible.)
Where he told is was a lunatic, crazy, mad etc. (and don't get me wrong, a lot of people are going to give you shit)
1908 comes by, and soon enough he becomes one of the richest and profound men of the 20th century.
By making a positive, stern decision to do so. And in comparison for your scenario, if you want something bad enough, take action to do so, and may be willing to endure hardship - then your rewards will be correlate to your actions.
You ask specifically, "What i really want to know is, how long would it take a 17 year old British below average student to become a video games journalist and would i even be ABLE to become what I've yearned for so long to be?"
How long? - As long as it needs to take in your own willpower and personal initiative.