Either can get you into a great grad program- Warwick will be well recognized by the good grad schools, even if your gran and next door neighbor haven’t heard of it. It will be down to what you make of the opportunities you have.
I suggest paying attention to @PurpleTitan’s point about the differences in the school systems. Are you good at independent work and self-motivation? When you need to be, can you be quite self-disciplined? Did you handle your college applications independently? Can you handle having a few very big assessments- such as a single exam at the end of the term determining your entire mark for the module? If you have doubts about any of these, UCLA may be a better place for you.
Also pay attention to the course itself. Remember that your choices for classes are much more limited than in the US. At Warwick, in year you will take:
Intro to Financial Accounting; Economics for Business; Quantitative Analysis for Mgmt 1 & 2; Management, Organisations & Society; Markets, Marketing & Strategy; Foundations of Financial Management; & Operations Management, plus CORE Practice and an Integrative Project.
You get electives (selected from a list of relevant topic areas, including marketing, strategy, operations, finance, accounting, HR, entrepreneurship and econ) along with Global Environment of Business, Governance & Social Environment of Business, Economics of Strategy, and a Global Integrative Project in Year 2. Year 3 is Critical Issues in Management and more electives.
Do those courses sound like a happy 3 years, or would like to have some other courses in the mix as well?
Finally, as a full-time student at Warwick you are eligible for (paid) internships in the UK, and all of the big name finance and consulting firms recruit from Warwick for those internships (fwiw, Warwick students were #3 after Oxbridge in the placements my collegekid and her pals have had).