Joining the RAF with a view to becoming an airline pilot is just about the worst idea in the entire world.
IF they accept you - and thats a VERY big if - you will spend 3 years in training acquiring only a few hundred hours. I doubt they will accept you though as they can sniff out someone applying with less than 100% commitment a mile off.
Do you realise that about 16,000 apply for every pilot job in the airforce? You will weep at the CV's of some of these people.
WWW