The 77W is not guaranteed on the DFW or JFK flights to LHR for now, since those routes are also served by 772s and an aircraft swap or schedule change is very possible.
So, unless very close to departure dates, I would not go out of my way to book a flight currently scheduled on 77W for any JFK or DFW route.

Since LAX-LHR has only one flight, I would feel more confident that once 77W starts servicing this route aircraft changes will be very unlikely.

Having said that, personally, I put a lot of value on my time in the ground and on my precious personal/vacation days. So I would not waste more than 8 hours just to try the 77W.