(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?
I would't be especially keen to change airports, especially as some part of your journey will coincide with the morning rush hour, doing this with luggage could be incredibly unpleasant... You can plot the route on google maps and see how long it takes. However, to get some fun trains and the full Japanese train experience you could take the monorail (cool ride over the water) to hamamatsucho then the yamanote line (busiest train line in the world) to Tokyo station before getting on the Narita Express. This ins't painless nor quick and involves navigating tokyo station.
Personally I find the JAL ground experience to be comparable at both airports. Transfer screening at HND is very fast.
There are no fast track lanes from immigration and depending on the arrival time the lines can take about 15 min max to clear, but generally you are through into the baggage hall before your luggage.
The JAL HND F Lounge has great views over the runway and the JAL domestic terminal. The food and booze options are almost identical between HND and NRT and at that time in the morning neither F lounge will be particularity busy.
Onboard, the crew from Narita will be WW crew whereas HND will be MF.
I much prefer the arrival time in London of the Haneda flight.