I have to agree, my Micra had been garaged most of it's 18 year life and from what I could see there was 0 rust, like, everything was completely solid. And when we cut it open, sure enough, there was some of the orange devil on the surface of the steel inside of the rear sills. Very minor considering it's age though and wouldn't have been a problem for a couple years atleast.
EDIT: However, if there's so little rust that you can't see it, I'd not worry about an MOT, I'd just keep it in mind that you might need some preventative efforts one day.