I would have warned against ever routing the trip EWR-IAD-LHR in the first place (that's always an option begging for trouble), but that's water under the bridge now. Above all else, highly recommend focusing on getting your friends on a IAD-Europe flight, no more domestic connections
Two Y seats JUST opened on UA918 IAD-LHR @ 625PM. Grab em now!
Besides that, I like the option of LH417/LH 924 (via FRA), dep. 1525 arr. 0740+1. UA agents are most accustomed to using LH flights as the first option for TATL reprotection when UA options are not available. True, the 325PM departure time is not at all friendly to one’s body clock, but the ultimate arrival time in London compares favorably with the later IAD-LON nonstops (and they get moving sooner). LH A330 on IAD-FRA is fairly decent in Eco. Importantly, the flight is showing 9 in many Eco buckets, so if it takes a UA agent some time to complete the rebooking, there’s a good chance avail won’t evaporate in the meantime. FRA's weather looks excellent for tomorrow morning.