I need help, im 15, how can convince my parents to let me go away for new years?

my gf's parents invited me to go, she lives in london i live in belfast, she was here last week and they want me to go to there new years party and she really wants me to aswell. i tryed to convince them to let me go, i wrote a letter explaining why, i tidyed my room, i did hw, i revised, her parents even said they would pay half of it and i would pay the other half. i also said i would walk to the airport so they dont hav to waste ther time driving me there and her parents offered to collect me from the airport. some of her friends and relatives want to met me there and i have told my parents this but they still wont let me.... what can i do?