LAX to LHR is currently the only reliable service option, I can't see anything else running. The LGW service to LAS is beyond aspirational at both ends. There may be a case to book it, knowing you will be moved anyway -but that brings a number of uncertainties to the game. You would need to self isolate on arrival in the UK for 14 days, however going to a funeral is one of the exceptions for the day in question, there is a long list of rules for funerals in the different parts of the UK, some locally derived.

The WTP seat on the 787 is pretty good in my view, particularly for day time flights, but the sides of the seat are non moveable. If you knew for sure that the WT cabin wasn't full - and typically they are not - then you would get multi seat occupancy, particularly sat nearer the back. Ideally you would go WT on a low loaded service from LAX to LHR, and WTP back to the USA.