Thanks to all for the very helpful comments. Looking forward to my trip and transits, plus a nice stopover in Japan on the way back. Great responses.

I am flying a new route on JAL codeshare Mel - NRT - LHR (codeshare BA) - NRT - Mel in J. I am QFF platinum. I am looking for advice on best lounges to use at NRT and LHR. I believe I can still access the Qantas First lounge at Melbourne as I am flying on a Oneworld carrier. Many thanks in advance.