Earlier quoted context omitted.
Shannon also has pre-clearance. The list is small outside of Canada though - Dublin, Shannon, Abu Dhabi, Aruba, Bermuda and Nassau.
You can use eGates in the UK, same as me as a British citizen (as long as your passport's new enough, probably all of them by now) - that covers it doesn't it? Otherwise I'm not clear what 'pre-clearance' is skipping? Painless for me into Canada too.
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.