What dep/arvl times do y'all have for LHR? Are they going to be late afternoon/early evening departures out of the east coast with morning arrivals into LHR with morning departures out of LHR back to the US late morning/early afternoon?
If that's the case, those schedules are probably the most commutable trips in the business.
Will y'all be staying downtown or out by the airport? If you're out by the airport, the overnight will get old very quickly.