Well, Oxbridge colleges give out EE offers on occasion, so its perfectly possible that there's plenty of AAB students there. I know of at least two on this forum who did not get AAA. The bigger question is, is AAA at A-Level any sign of either quality or future potential? I don't think accepting people from certain backgrounds who acheived AAB would 'dilute' anything, indeed it may be the case that those results might indicate more potential for future academic prowess than straight As in from other people in other circumstances.