As others have said, stick to your education for now, University is not essential IMO but at least to A level, then try for any rail job to start with to get a foot in the door, but if you don't succeed, then for conductor try and get some customer facing experience as that seems to be a strong point the TOCs want nowadays. If you're looking to become a driver, engineer or electrician might be a good start, it's something I wish I'd looked at, not because I want to be a driver, but because it's interesting and I imagine would be helpful in understanding traction training. Finally, be prepared for the long haul. Like you, I knew at 15 what I wanted to do, but it took me 10 years after leaving school to get there, along with countless applications (probably 30+), multiple interviews, etc. You'll want to give up and question if it's worth it, but once you're in, it is!