The reason why, actually, is because you are partaking in the recent MR deal, the LHR-IAD segments are BMI codeshares on UA metal. UA blocks out only a few seats in E- for codeshare seat assignment and assigns the rest on the day of the flight (to give UA customers first shot at the seatmaps). It doesn't matter if you are an elite, .bomb won't let you access the UA-partitioned seats.

The only way around this is to call an agent directly. Since the MR deal is now dead, it's considered OK to call a live agent and add your M+ number, get seat assignments, etc. Hope this helps!