Why can't I get braces from the NHS?

Today I went to the orthodontics to enquire about braces, and the orthodontist i would need a standard fixed brace and i would have to wear retainers forever behind my teeth on both rows becuase of the gaps between literally all my teeth but the front 4 on the bottom row. But she also said the NHS cannot fund it. I would have to go private and pay up to 3 grand! So I went home and went on the website, and it basically said you cannot get braces funded by the nhs unless your case is major or if your teeth can lead to health risks. I cant see how my case is not major becuase im going to have to wear a retainer forever! Also it says I'm a grade 3 on the scale. Also the orrthodontist said i have a 4mm overbite and i have an over jet too. I just want my teeth fixed ever since i was a little girl. I used to say to my mum i would get pink braces on my teeth. Back then I even knew my teeth werent right! By The Way I'm 13 now. Help!