topcat, you said "US sats are not an equivilent to A level, which is why Harvard et al ask for 5-6 AP courses passed at a high level as well as very high SAT scores, but offer places based on 3 A levels to students here."
It is true that SATs are definitely not an equivalent to A level - frankly they are not very challenging and, in the big picture, not very important. You are incorrect about the APs - decisions are made during the senior year in the US and are not based on predicted grades, but transcript of all grades in high school, national exam results, and other factors (the extracurriculars). At the time of decision making (either early December or mid April, AP grades would not be known, nor would A level results so these are not the deciding factors.
For the top liberal arts colleges and unis in the US, including Ivys, grades and test scores are just the sorting factor to even be looked at. The acceptance levels at these schools hovers at the 5% mark - the deciding factor for these schools are the extracurriculars. I have the list of candidates from the UK who will be entering the freshman class of Harvard to hand and some background info on them. Half of them are international athletes, one is a IMO medal winner, and several are internationally recognised musicians. That makes up the majority of the incoming freshman class from the UK. These children have these accomplishments as well as top marks. The criteria is similar for the other top US colleges. Much easier to get into a top UK uni!