OP,
As stated above, you will need parental permission to travel outside the USA, also since you are considered a minor, the airlines may have further requirements as well, so you should check with each airline as well.
Entering the UK, I would suggest having a certified letter from the parents of your girlfriend inviting you to stay at their home as well as the contact information in case the UK Immigration Officials want to call and confirm this personally. Also, if something were to go wrong with the relationship, and they ask you to leave, where would you stay while awaiting your return to the USA. Also they may ask for some proof that the relationship was initiated in person, and is not one of communication by internet only. This might include pictures of you and your girlfriend.
<off-topic commentary removed by moderator>
Last edited by JY1024; Apr 9, 2017 at 7:10 am
Reason: Off-topic commentary removed by moderator