For math, Imperial might’ve been a better option than UCL.
Bristol is roughly on par with Tufts; for undergrad, I would prefer Tufts. I placed UCL above Tufts, yes, just as I would a number of research universities here in the US (Michigan, Wisconsin, Berkeley, UT Austin, Johns Hopkins, etc.). Keep in mind that I am only speaking of overall academic breadth and depth – not the quality of undergraduate education, which is much more difficult to measure or compare.
Many people consider Tufts a happy medium between research universities (lots of exciting research and classes but can be large and impersonal) and liberal arts colleges (which focus all their resources on undergraduates but can feel suffocatingly small after a couple of years). Along with its Boston location, this is arguably its biggest draw.
EDIT: The league tables are as follows:
St Andrews: #5 Math, #44 Econ
Bath: #11 Math, #30 Econ
UCL: #13 Math, #8 Econ
Bristol: #27 Math, #14 Econ
http://www.theguardian.com/education/ng-interactive/2015/may/25/university-guide-2016-league-table-for-mathematics
http://www.theguardian.com/education/ng-interactive/2015/may/25/university-guide-2016-league-table-for-economics