I assume Imperial think that a more focussed PS would be what they want for their course. However, that wont do for Nat Sci - where you need to show very clearly that you have a wider interest in all sciences and in particular, the overlaps/connections between them. All Nat Sci programs are slightly different because they rely on the choice of subjects offered at that individual Uni, but some will include Geography, Technology, Psychology, Management etc units as well as the conventional sciences like Chem. Bio, Maths, Physics.
Nat Sci is a useful degree - you would learn several different 'sciences languages' and get to see the wider picture, and the multi-science solutions to problems. These are exactly the sort of skills that employers want. I'd advise you to go to as many Nat Sci Open Days as you can and see whats on offer at that Uni. You can talk to current students about what they enjoy about the course, and the Uni can tell you about placement opportunities and the sort of careers their graduates have gone on to do.