The GWR has more than traction to aspire to...history, heritage, route coverage, helping local communities transport goods from the SW of England to London (what a number of other TOC don't do!), a varied mix of work. High speed, local suburban stoppers, branch lines (using old historic systems that again a lot of TOCs don't use!), operating trains in both AC/DC areas, old signal boxes, new advance signalling centres, absolute block semaphore areas, list goes on!
As for the traction types a number of operators use them. SWT, Southern, LOROL, South Eastern and many more. You don't have to be internal to get a job on the railway. It can be an advantage if you have knowledge of the railway (by working as a gaurd) but not a must. As many others have posted, look at the skills required to do the job and gain experience in those areas.
As EWS has mentioned, forget 21 being a target age. In fact don't even target an age. Gain your experiences and apply when your ready. You'll be left disappointed otherwise...