84fiero

Neither of OP's airlines (nor any that I'm aware of) has any special requirements for a 17 year old flying alone...mandatory use of the airline's unaccompanied minor services ceases at a younger age than 17.

Basically what Often1 listed are the main things I would think of. Review the baggage allowances for your airlines - assuming coach, would be 1 checked bag for free. Same carry-on liquids restrictions as when flying domestically.

Are you asking about a layover in PHL on the way home? If so, yes you pick up your checked bags and take them through customs (after passing through immigration), then after customs there will be a bag drop for your checked bag - you just hand it off to the airline there.

Since you're planning separate tickets from Alaska to SEA, be sure you have plenty of buffer built in between that and the start of your ticket to/from the UK. Otherwise if your separately ticket flight is late and you miss the next (separate ticket) flight, you're SOL as the airline would consider you a no-show. While sometimes you might get very lucky and the airline would get you on a later flight, they are under no obligation to do so and you definitely should assume they won't do that.

You'll have to pick up your bag from the Alaska-SEA flight and re-check them with the airline on your separate ticket from SEA. Personally, I'd arrive to SEA the night before or at worst, very early in the morning if your departing flight is late evening-ish. I might be willing to cut it a little closer on the way home, depending on other options to fly back home from SEA (and the cost of purchasing such on a walk-up basis).

Also, just plan on withdrawing cash from ATMs in the UK (using a debit/ATM card not a credit card) and don't be concerned about changing money before you leave the US.

You might want to look into trip insurance as well.