[Allworth]

Structural parts are made of steel.
Steel is very prone to rust when untreated, normally some sort of very heavy oils are used to protect a metal. This protective layer has its lifespan and tends to harden with age which makes it easier to accidently damage it and take a piece off. Once metal is exposed it will start to corrode, as been mentioned before - salt will accelerate the process. Due to subframe location on the bottom of the car - it will more often come in contact with road dirt and salt and moisture. As your car been in Scotland that is known to be colder then the rest of UK, I could assume more salt is being poured on the roads.
It think it is a good idea to have a car wash regularly during those salty days.