DC have know a number of students who have gone to the US to study.
Interestingly several good scientists/economists have opted for the UK for undergraduate with the intention of continuing to the US for postgraduate because of the greater specialisation the UK can offer. (Though this can cut both ways with strong mathematicians etc deciding the US offers more scope for allowing them to keep up with other interests and talents.)
The breadth offered by liberal arts certainly suits some.
The reputation of the University appears to matter, not just internationally where only a small number of names are familiar, but in the US.
Competition for top name Universities is huge. Higher than for Oxbridge. They will be looking for a load of achievements, beyond academic. Some London children appear to start building content for their personal statement very early, indeed at Primary.
Motivated kids start practicing SATS early. They can be taken repeatedly, and are, though each time costs.
Entrance requirements for each University are gruelling and it is common to apply to as many as 10 Universities. You need to be organised over the summer otherwise it will conflict with Y13 preparation for A levels.
England's early specialisation can give problems when applying to the US, you need to choose your A levels carefully. (Though design may be different.) IB is often seen as a better preparation.
There are some expensive consultants out there. American friends suggest that they do not always deliver on promises and can be very expensive. (A friend suggested that one, who spoke at an event I went to, charged in the region of £10,000, just for help with applications.) If you want to learn more about the process, American Universities themselves make recruitment visits to the UK (or at least London) and details are presumably available via the Fulbright Commission or similar.
The advice is that it is normally much better to find a sixth form with good experience of American applications, either an international school like Southbank or ASC (both offering IB), or an academic private school like Westminster (whose Early Decision hit rate is often extraordinary - even by American standards) or SPGS, or a sports school like Millfield (or Kings Taunton for soccer) if you are looking for a sports scholarship.
We are lucky in the UK that we have a number of good universities offering a range of interesting degrees. The best US Universities can offer amazing opportunities, but unless you are exceptional, in which case financial support may be available, or rich, or have strong family links with the US you want to foster, I suspect studying in the US is not worth the extra cost.