Point of order: I don't think UA does any Europe flights out of DEN. NRT is the only intercontinental+international UA flight out of DEN. (You could obviously do YYC-DEN-xxx-Europe, but it sounds awfully painful). There is service to Europe, but only on LH, BA, and FI.
Good point. ORD it is, or just take the AC or LH nonstop on to LHR or FRA.