Right get this straight. Stop worrying about money so much in deciding an aspiration like this! You know it is well remunerated. You don't need to know the nitty gritty at your stage. Ask questions like that at an interview and you will get no further!
To answer your other points- it takes a massive amount of work to fly one type, so one type only you will fly. do some research at BA's route maps to see where you could go. It is extremely unlikely you would start on a 747. You would do many years earning your wings and experience doing hard graft before flying the glam longhaul stuff if you were lucky, where you would then find life hard and extremely tiring.
You either know if it's for you or not. At the age of 15, the money should be of not overriding interest!
Without being rude or cruel, concentrate now on your education. Get your maths and grammar sorted out so when you do apply, your application to whatever you are after is sensible and your education shows you to be a good and qualified candidate. Remember the competition will be hot!