I wouldn't beat yourself up about study on aeronautical engineering. They have offered you an interview, primarily. on the basis of your CV? If that's correct they will clearly see you have had no previous aeronautical engineering experience and won't expect too much from you on that score.
As I've said previously, just be yourself. Find out as much as you can about Marshalls and show you are a potential willing apprentice who will benefit Marshalls.
Again, good luck!