Si,
You are right, they cannot score an 18/19yr old with any accuracy, unless you have defaulted on a loan or the like.
A CDL is underwritten by the Government so the Bank are not at risk, although they do have to demonstrate that as agents for this Government loan scheme they are acting responsibly.
I would give the Royal a ring and arrange to see them, get them to fully explain their rejection, it sounds a bit strange to me.
Having said that, the Banks want to be sure that you can complete the FULL course of training to the point where you have a vocational qualification and can earn enough to pay back the CDL and its exorbitant high rate of interest!.
Now that is where you might have fallen down. £8k won’t let you complete a CPL/IR from PPL…by a long way. They will want to know that you can fund the remainder, this might require a guarantee from your parents etc etc.
All the Banks who offer the CDL operate under the same rules, but it might be worth giving the Clydsdale a go.
Good luck