As said above getting a job as a Driver in a TOC is far easier than getting one in a FOC. Once your in a TOC you can move to a FOC later.
Regarding the Internal vs External debate (with a TOC):
Getting a job as an Internal then becoming a Driver is easier than trying to get in as an external - depending on TOC. Getting in is *the* hard part. For me getting in Northern as Platform Staff was hard and I got lucky to get in. Once I was in it was relatively easy to secure a very good Drivers role as an MDD, I had a few knockbacks while I was very new but I knew I would be going Driving sooner rather than later. Now I'm driving as an MDD there's countless opportunities to move into other forms of driving (i.e. mainline) but I prefer to be doing what I'm doing now.
I'm not in the minority. Loads of people join my TOC as Guards and become Drivers in less than 12 months. It's a proven path. Some have even joined as Cleaners and are now Drivers. As an external, to get the best chance, apply for drivers roles and other roles as well, such as Conductor, Platform Staff, Cleaner yes some of these jobs are hard to get in as it is, and they require different skills to driving (one can be good at one of these and terrible at driving, & vice versa) , but once you are in, its far, far easier to move around - but you need to be in a TOC that recruits trainee drivers in mass numbers like Northern for example. If you are in Virgin for example then there's almost no chance internally; they mainly recruit qualified drivers, in low numbers.
It may be worth trying to attempt the psychometric tests as soon as you can because if you cannot pass that then Driving is out of the question.