@exlibris97: Yes, Oxbridge admissions is by the colleges, though in at least some courses (I believe physics and math at Cam and/or Oxford), the faculty try to make sure there is no variation of standards across the colleges.
And yes, more than marks are considered (especially by Oxbridge), though the key point is that the evaluation is being done by faculty, not adcoms, so they would naturally be most interested in academic potential and in any case, for a full-pay International (like an American), if you meet the requirement for marks at any UK uni outside of Oxbridge and LSE, you’re likely in.
That makes admissions to most UK Unis much more certain than admissions to any elite in the US (for an American).