Personally I'd say Warwick. The MORSE course is *amazing*, it's very well taught, very well run and co-ordinated between different departments (very unusually for a maths/econ course) and extremely well regarded by employers and academics. I'd even say it's Warwick's flagship course. Whereas while Imperial is utterly amazing, it's best regarded for it's applied science, not it's maths or undergrad business.
To be honest, I think they're two good options, and neither will shut you out of any career path whatsoever. Imperial has the edge on reputation, but IMHO MORSE is a better taught and set up degree. The question has to come down to: Do you want to do economics or management? Would you rather live on campus in Warwick or in London at ICL? And the last one, can you afford ICL? (South Kensington isn't cheap, I know I'd have trouble affording uni in London). But smile that you get to choose between two amazing options.