This. Daytime flights = no need to sleep in economy which for some people is impossible.
However, there are no daytime flights from the West Coast to the UK (LHR). United and AA have daytime flights leaving from NY, AA additionally offers one from Chicago and United one from Washington. All these require either an overnight stay (LAX-NYC the day before, one night in a hotel) which significantly prolongs trip time or an uncomfortable redeye flight to the East.
If going the upgrade route, I'd look for AA/BA LAX-LHR flights. both allow upgrades from the lowest fare classes and due them using aircraft with large premium cabins on this route (the BA A380 offers 97 business class seats) there is usually upgrades available.
See the BA POUG thread Pro-active Online Upgrade (POUG) tracking and discussion | 2024 thread - FlyerTalk Forums
AA's MCE is the cheapest way to ensure a little more space, but all that gets you is additional legroom, otherwise this is a standard economy product. Premium Economy has wider seats and more recline, but still cannot compare with a business class bed comfort-wise.