Mathematics, English and Science based subjects will stand you in good stead. A levels more so, but pick them in subjects you are interested in - as long as they aren't Media Studies or General study type subjects.
However, it isn't all about your qualifications, you need to have the right aptitude. Also, most importantly it's all about teamwork - can you work in a team? Are you a good leader and a good listener?
As someone said above 'The best way (and easiest) is to join the RAF' - I wouldn't totally agree with that, I'm not saying it's wrong but there are 'easier' options. Bearing in mind how competitive the RAF, FAA and AAC with their Pilots and Aircrew.
That's not to say you can't get there though, with hard work and determination, you can succeed.
Sadly, it's a financial killer - it is expensive and unless you've willing parents who are happy to remortgage the house or you will on the EuroMillions, it'll take a while - but again, it's about determination - are you willing to spend X years training and getting all the licenses required? That's what the Airlines will be looking for - someone who has a strong work ethic and is prepared to keep going even when it gets tough.
I'm not an Airline Pilot or indeed a Pilot, however I do work in Air Traffic Control and a few close friends fly for Airlines so the info I've given is as good as my knowledge goes (it could be wrong or need updating).
I'm currently in the process (3/4 of the way through) of applying as an Aircrewman in the Fleet Air Arm (and previously say the RAF Aptitude Tests at Cranwell for the role of RAF Weapon Systems Operator) so if you want to ask any Q's reference that - feel free to PM.
Best of luck
Posted from TSR Mobile