I think it was 2010 when the grade of Depot Driver was introduced at Derby Etches Park and Nottingham Eastcroft. I think the first to progress to Main Line driver was in 2012 and went to St Pancras. I've heard a rumour he has recently been offered a job with Eurostar?!
There have been a few others progress over the years, but there are some who are still Depot Drivers.
As I said earlier, you don't have an automatic right to progress. If you apply to go from Depot Driver to Mainline Driver, you will compete against all other applicants (I.e. it won't be an automatic promotion based on seniority), and your 'Safety of the line' record, experience, attitude, attendance, and interview etc. will all be taken into account along with all the other applicants when the company make a decision over who gets to progress to Mainline Driver when a vacancy arises.