Have to agree with Scroggs here.
The services really won't take kindly to you using them to get into the airlines (though I know several friends who have), and your commitment to the services will be tested to extremes long before you get anywhere near an a/c. For the RAF, initial officer training is something like 18 weeks (assuming you pass first time), and it is hell. They will push you up to and beyond where you think your limits might be with good reason.
As for the flying itself, yeah it seems fantastically glamorous and all very Top Gun, but the reality is very different for most. Most fast jet pilots (current situation excepted) spend very little time in the air each month as a result of defence budget restraints, and if you have ever spent any time on an air force base, it is the absolute pits. You really don't know boredom until you have spent endless weekends living in a room in an officers mess at an airfield in the @rse end of beyond. The lifestyle really isn't everyone's cup of tea.
Finally, you might feel quite at ease with the idea of operational service and the prospect of having to kill or be killed. I always used to feel that way when I was your age and wanted to be a fighter pilot, but when the last Gulf War kicked off, I was involved with the RAF at the time and it REALLY brought it home to me just what was involved, and it is incredibly sobering. Some of my friends from those days are serving pilots with front line Tornado squadrons who have seen service in the Balkans and the Gulf, and have been shot at, and have also dropped bombs on real targets. While we can all laugh and joke about it over a beer, it is a serious business and they are only too well aware of the risks to themselves and the consequences of their actions if they get it wrong.
I think it is great you want to be a pilot so badly and I applaude you for having the foresight to ask so many questions so early, but if 747's are what float your boat then the RAF/RN/AAC really wouldn't be your best career choice. However, you might find out that once you know more about service life, airlines seem about as exciting as driving a bus for a living (everybody would rather be a rally/F1 driver wouldn't they?), albeit they seem to have got the pay muddled up.
Fortunately you have time (and enthusiasm) to figure it all out, so best of luck wherever you end up.