Check out the employment prospects of Warwick and Southampton, try to talk to their alumni and visit their campuses to get the student vibes. Choose based on what you have gathered.
Generally, I prefer to go to the uni that has a lower drop out rate to ensure my graduation prospect.
Based on university reputation alone, Warwick is better. But there is more to it than university reputation especially if we're talking about fields in engineering. That said, Warwick engineering looks promising as the university has infused a lot of funding to boost that department.