You cannot become a train driver in the UK until the age of 25. Also, you do not need to have any formal qualifications. What you do have to do - having had your initial written job application accepted by a Train Operating Company - is to pass a very stringent day-long assessment which will examine a lot more than your technical knowledge and physical fitness. This examination also checks your psychological make-up, tests your ability to accurately carry out boring tasks for long periods, your physical co-ordination, and much else. There is very little that you can do to practise or revise.
The TOC's vary in their employment policy for drivers: Some prefer to recruit from existing staff, and those that 'came up' this way on the TOC I worked for were ex-conductors, station staff, revenue protection staff and technical maintenance staff. Other TOC's prefer to recruit 'off the street', believing that such recruits are less-likely to have ingrained bad 'railway' habits. Such recruits tended to come from occupations and professions that involved responsibility and demonstrated the ability to work on ones' own, such as the armed forces, police, prison service, self-employed. The company that I worked for was in between the two extremes: I was forty when I joined the railway as a trainee driver, having spent twenty years as a self-employed freelance illustrator and designer. Other new drivers that they took on included such diverse previous occupations as a bank manager, an airline pilot, a casino croupier, and a self-employed lift technician!
I hope that this will be of use, and I wish you the best of luck. I spent twenty years driving trains - suburban and main line - and enjoyed every minute of it.
Retired UK Train Driver