I haven't seen that film so I can't comment ! I'm not sure that I've seen any film that Hugh Grant has been in - possibly just the one about Byron when it was shown on TV, several years ago. I thought your term 'monument' meant 'Ancient Monument'. Mix-up.
The inch is the width of a male thumb; the foot is the length of a male foot; and the yard is the length of a male stride. Roughly. So the difference between a hill and a mountain is quite subjective. I'd say that if the top collects snow - in December - it's a mountain.
When does a stream become a river ?