Williams, while offering a fantastic liberal arts education, does not possess the prestige that other Tier One colleges/universities do. I would say:
Tier 1
UK: Oxford, Cambridge, LSE
US: Harvard, Yale, MIT
Tier 2
UK: UCL, Imperial, Edinburgh
US: Columbia, Duke, Caltech, UC: Berkeley, Brown
Tier 3
UK: KCL, Warwick, York, St. Andrews, Durham, Manchester
US: Cornell, Northwestern, Dartmouth, Johns Hopskins, Georgetown
Hopefully no one kills me, since I am obviously going by what I have heard (prestige and reputation over actual quality). I don't think anyone here has been to more than 2 or 3 of these unis to study.