Physics at Imperial is an astounding course, you really cannot go wrong with it. Besides, you can still reapply as a postgrad for your doctorate if you would still like to study at Oxbridge. Personally, I would not look a gift horse in the mouth and take the opportunity handed to you. As you well mention, there is no guarantee that Imperial will accept you the following year, especially as it would seem that they were your second option all along. Any sort of mathematical or physics based course at Imperial is world leading, as such they do not have time for people being indecisive over league tables. In reality, Imperial and Oxbridge form the golden triangle of STEM universities within the United Kingdom and are well respected around the world. I would not worry too much about their differences, they are bound to be ironed out in postgrad.