Im going to go against the flow and say yeah the earlier you plan something the further ahead you will be compared to the guy stood next to you who planned a year or two later.
You can start your training for a PPL as soon as you can reach the pedals, the current record for the youngest person to fly across the atlantic is 6. You do however have to be 16 years old to fly solo and 17 years old to be awarded your PPL and take your skills test. (Kind of like your driving test)
Get good GCSE grades, don't worry about choosing A-levels until you have done your GCSE's as you will deffinitley know by then which subjects you are best at. Go with the flow, have an idea but choose subjects you will do well in not what you think will look good on paper. That being said some courses required A-level Maths or Physics so if they are your strengths strongly consider them. If you plan on going to university most degrees will vouch for your academic ability to pass the course.
The reason people say go wash planes on saturdays at 7AM is because it shows motivation and sounds great in an interview. You do not need to go overboard. Choosing to do your work experience at a local flying club and getting some flight training under your belt shows motivation without having to work for peanuts during your weekends. Your spare time can be used just as effectively taking part in clubs such as sports or completing a duke of edinburgh award.
At the end of the day your trying to avoid the question, so why havn't you done anything towards achieving your goal of becoming an airline pilot?
To answers your question about experience. So far the only cap on flying experience I have come across is maximum 60 hours which is plenty room to fit in a PPL minimum 45 hours required. That is an addition third of your total flying time you can do before they think they have to bash bad habits out of you. Despite that 60 hours requirement there was a candidate at one of the sponsorships I applied for with 78 hours well over the 60 limit.