You will find it difficult to be a train driver in the uk at 20, many of the companies will require you to be 21 to drive a train. Also depending on where you live you will have to be within 20 - 30 mins travel maximum from any depot that have drivers, so again you would have to enquire to the local TOC to you area.
I would suggest that you look for training on the railway as a Conductor, Guard, Platform Staff to tide you over until you re old enough as they will give you some understanding in the Rules and Regulations of the Railway. Conductor / Gaurds only have a few more rules to learn to becoming a Driver, Platform Staff have a little way more but learn the basics like PTS (Personal Track Safety) and Basic Safety Critical training.
Plus also you will recieve the internal job applicational forms and have a better chance to see where the Driver vacancies are and prove to the TOC concerned that you are dedicated enough to become a Railway Employee.