(Bumping from January...)
I've got a variation of the OP's query and wondering if anyone would care to share their opinion.
I'm flying back from SYD with SYD-HND in QF J, red-eye landing at 0530, followed (currently) by HND-LHR in F on BA8, which departs at 0850. But I see there's availability on the BA6 789 from NRT at 1235.
Regarding the time difference or hassle of changing airports - I hope to be well rested, jetlag and arrival time at LHR aren't really a concern, and tbh a morning spent on trains through Tokyo doesn't sound like a bad way to spend my time! What I'm interested in is opinions as to which combination of ground and cabin experience will be the "best" in F.
On the ground: I've never flown from HND and only once from NRT in 2006, in J, so safe to say I'm unfamiliar with any F lounge(s) at either airport. I've read the lounge database but it's not obvious to me which lounges are better (by which I mostly mean: quiet, good food, plentiful booze).
In the air: I've not flown F in the 789, but have in the 777, 747 and 380. Is it a noticeable step up? And will the service be different on the early morning vs early afternoon departures, or with HND vs NRT crew?