It's much more to do with their slot availability at LHR. The pmUA flights into T1 have one set of slots and the pmCO flights into T4 have another set of slots, as I understand it. A pair of slots is an arrival time and a departure time. UA has to schedule its arrivals for those times. Presumably, they have chosen to run two IAD flights at similar times, rather than using the first or second time for a flight from ORD, LAX or SFO, and switching one of those to the IAD time. That decision probably does lie with the timings at the originating hubs - or perhaps the return times are the decider there.

In any event, UA's slots are differently timed when the clocks go back so they'll be a different timetable in place then.