Cohort 11 scholar here, context is always important as you pointed out and strong EC's can defiantly make you a compelling candidate to Sutton and US universities if you know how to market yourself (Grades are much easier in that respect). The initial application to the program is trying to mimic what your US application will look like come year 13 so learning about how US institutions will read and understand your profile is important even now. There are great resources online but many wont map neatly onto your situation so proceed with caution.
VERY strong can mean lots of things, but showing impact is what everyone (Sutton/US uni's) is looking for, if you can weave a cohesive narrative about yourself (identity, grades, ec's, work, family life, financials etc) you make yourself someone that the reader can see attending their institution/program.
Wishing you the best with your application.