Its hard to understand what an engineer does while still at college imo.
I'm studying a BTEC in engineering, the stuff we are studying is pretty much the foundation for a Mechanical, Electrical or Material science engineering degree.
There are no experiments, no working of parts or fixing cars even though we have a whole department dedicated to practical mechanics. The only tools we have learned to use so far are drawing boards. I have sat in lecutures at university and have some friends who study and have jobs in engineering.
I suspect that science and maths is made up of such a wide unspecific number of subjects at GCSE and maybe A level its hard to know what to study. Not knowing what engineering is, people might go on to study physics, maths or chemistry when actually engineering is pretty much physics, maths and chemistry combined with many other subjects. Its also the in thing now for some to not do some thing that leads to a job, but something a teacher or some one feels you are good at. Subjects like maths and physics are so subject specific they are only useful for the knowlege you gain studying them. For example statistics in maths. People who become physisists or mathematisions are only the eleate.
Another point is the image of an engineer might not be as good as say Asian countries like China. I heard here almost every one is being pressured to become an engineer or other high payed job which benefits the economy.
In England the social pressure is to have the most fun you can and spend the most money you can. This might be one reason why people might not be thinking about what happens after graduating.