>>12439889"The college focuses exclusively on science, technology, medicine and business. "
STEM is Imperial's focus on and they outrank UCL in terms of math. STEM is basically what they are known for so they beat UCL in terms of reputation and research output for those subjects.
If you are a social person who values a vibrant student life go to UCL. You won't be motivated if you're miserable socially. Not to mention that you also want to study economics
It's a math undergraduate degree so those things don't matter as much especially if you have 2 excellent unis. This is not a one way street provided you have the necessary grades since masters degrees exist at Imperial & Oxbridge