It was announced as a daily year-round route beginning Spring 2008 once Bermuda II was rolled back, though with oil and the economy, it was cancelled promptly in October. It came back again as a seasonal route in 2010, only to not come back the next year. Though it should also be noted that 2010 was around the same time the UA/AC/LH joint venture kicked in, and when UA cancelled DEN-LHR for good, LH immediately started bringing back the 744.

The problem with UA flying DEN-LHR is that they're almost 20 years too late to the party. BA has been serving DEN nonstop without fail since September 1998, so of the regular/business traffic on that route, BA is already the established and preferred carrier. (Quite a sign of how weak UA was in the 2000s). I don't think there's any other destination in Europe that UA could reasonably support nonstop, other than of course FRA/MUC for *A reasons.

I could see UA trying DEN-LHR again on a 787 if they ever have enough to go around (and enough slots at LHR). It seemed like a 3-class 772 was the wrong plane for that route.