I am in the same situation as OP, choosing between Imperial and Soton for Aero Engineering and also on course to get A*A*A*A.
The problem is that I want to do a course that includes spacecraft engineering and while Southampton is great for this, Imperial does no spacecraft at all. Also Imperial is expensive to live in and I've heard a lot about the course being poorly organised.
I want to do a PhD after university and then go into research. Will there be a long term difference between a (probably 2:1) degree from Imperial and a (probably 2:1 or 1:1) degree from Southampton?