I know plenty of people with degrees, and a few engineers and chartered engineers that are in the railway as both drivers and conductors. Neither their education or my own was seen as a problem, in fact it's often taken as evidence of being able to sit and learn new info whilst in a classroom and then performing bookwork at home. A lack of higher education is in no way a sign of who will perform well when learning though, common sense and the willingness to listen, learn and follow instruction are far more important.
The engineers I mention are guys from the O&G industry who made a lot more money abroad and offshore but wanted a better work/life balance by being in their own bed every night and able to spend time with their family.
Coming from that industry it shows a knowledge of safety critical working and I suppose it also shows they're not in it for the money as the salary for a qualified driver is less and they also now have to pay UK tax.
Perhaps you simply didn't come across well in the interview. Perhaps they were indeed unprofessional and did take a disliking towards you for some reason. No matter what the reason for rejection, your achievements will be on your application so unless it's relevant to the question they ask I wouldn't mention it and I never needed to when I secured a driver's role. Being confident when answering questions in an interview is key, a little too confident may be construed as cocky.
All the best with your future endeavours, most people take at least one attempt at DMI before they land the job so don't let it phase you.