Both lie-flat business class products are about the same.
The United 777 from IAD-LHR is sometimes swapped out for a slightly different kind of United 777 which has the "old international business class" product, which has reclining seats that are more like Barcaloungers and does not have the nice new lie-flat beds. This can be a problem on an eastbound overnight flight (when sleep is a big deal) and is less of a problem on westbound flights if you're planning to stay awake during the whole flight. This equipment change can happen at any time, even hours before departure, until the entire United 777 fleet is completely reconfigured with the new business-class product (which should be happening soon -- not many old 777s left). In the rare event that a 777 swap does happen, SOP is to give you a $350 voucher to say "sorry about that".