Can I visit the uk at 15 years old?

I'm a US citizen and In May i plan on visiting my girlfriend that lives in the UK. I will be flying alone and will be staying with her. She lives with her parents and they have agreed to let me visit. My mom has also agreed to let me go. How long will i be able to stay in the uk and will they let me through border control at the airport? ( I'm going to get my passport and all the papers I need before I leave )

Hawaiian Airlines Baggage Size Leeway?

Hawaiian Airlines specify that the dimensions of hold luggage should not exceed 157cm when added up. The sum of my suitcase's dimensions however is 164cm. I've asked them and they say I could face oversized baggage charges of $100 for each flight I travel on, just for these few little centimetres. My questions are: how do they even know if a case is mildly oversized? Has anyone ever gotten away with oversized cases with Hawaiian?