BA48 leaves SEA at 18:15 and is scheduled to arrive at LHR at 11:25. BA196 leaves IAH at 20:35, at which time it is 18:35 in SEA, and reaches LHR at 11:30. In other words, very similar flight times and timing. You'd have to leave SEA several hours earlier to travel via IAH -- and would either have to do it on non-OW carriers or connect through another airport (e.g. LAX, ORD, DFW) that also offers direct flights to London. Sounds pretty daft to me.

As for getting "a full night's sleep": only if you skip dinner and can drop off at around 21:00 (which would feel like 19:00 if you were acclimatised to Seattle time). Or four hours earlier on BA194, of course.