[jen20]

Americans can _often_ use eGates into the UK - but the same set of exemptions apply as to British citizens using them. For example, if you're traveling with children under 10, you can't.

However, as others have pointed out, this isn't what pre-clearance is. It's more akin to clearing French passport control and customs in Dover or rather than Calais, and can make the trip substantially shorter than it might be otherwise.

When I still lived in the UK, I found it quicker and more pleasant to fly from Heathrow (or Bristol) to Dublin and then on to the US having cleared immigration during the layover. It's also quite a bit cheaper when you fly from the UK in a premium cabin since Ireland doesn't have the egregious air passenger duties that the UK does.