Thanks Lonli_Lokli That makes more sense now, but unfortunately the companion voucher may not work you for you in the most efficient way for what you have planned. You would have to fly back to London on a separate ticket and the. I would definitely nightstop before a BA /IB flight from LHR or the regions depending where you live to mitigate any risk of missing departure. If you don’t want to fly via HND either I would suggest use the likes of google flights to see if there’s a decent multi city fare available meeting your requirements of airports and cabin and save the companion voucher for another simpler journey ex UK.
As others have mentioned p, separate tickets bring risks in terms of disruption, so it all depends how risk averse and cost conscious you are.

I totally fell in love with Tokyo and Japan on my travels last year and yes train travel is easy and fast, consider purchasing the JR rail pass before departure as it can be extremely cost effective on the Shinkansen, I got my 7 day pass costs worth within 2 days of travelling q

i hope it will work all out for your travelling party