"It always amazes me that those enrolling think it is all about aeroplane design. It is not. 90% of the course is mathematics":
While what you say is in essence correct, I think the better way to phrase that is mathematics is a major element to aircraft design (well any engineering subject really). To add Aero Engineering is actually more challenging than a maths degree, in engineering you have to learn how to apply those mathematical principles to the physical application (by definition that is what engineering is).
Design without calculations is product design (designing fancy desk top pencil trays and such forth) not engineering.
You are however correct that many studying aeronautical engineering do fail to realise that it is just a variation on mechanical engineering. Also the fact that most of the content is mechanical engineering, means that it is equally useful for gaining employment in engineering sectors outside of aerospace, which considering the potential layoffs at RR, Boeing and Airbus means some Aero Engineering graduates will have to look to other sectors in the short term.