Even though checking in at UA Business, you still will need special access card for premier security line (which UA check-in should issue to you). Your BP x IAD is in economy and the checker at the head of the elite access line at security has no way of knowing and could never have the larger issue explained. That said, the premier security line at IAD is only marginally faster than the vast hordes. You are far better off at the Double Diamond line behind baggage claim one level down or on the other end of the terminal near DL. The UA side is crowded, backed up and depending on time of day, may be so backed up that elite or not, you stand in line to get in line and the elite line doesn't help you there.