I agree with you,
@gymtonic. Like you, I have flown with JAL from Sydney to Europe with a layover in Tokyo which can be up to 24 hours. You arrive before dinner so you have a free dinner at the hotel, then a free breakfast in the morning and stay in a lovely room free of charge at the hotel near the airport. In your bag which you take on the plane, you put the clothes you are going to sleep in, your toiletries and a change of clothes. Your suitcases are left on the plane and arrive at your destination with you. We had a choice of final destinations after we had a short layover in Helsinki. We chose Amsterdam as we had arranged to pick up a car there. But you could choose other destinations including London. At Helsinki, cases are moved to your next flight. As it's with the same airline, you do not have to pick them up.
IMO it is very likely that MB met RB in Tokyo and travelled with him to either Amsterdam or London. As Marion seems to have been going overseas for a year, it is very likely that one case contained summer clothes and the other contained her winter clothes.
It is probable that whatever airport they arrived at, RB suggested to MB that she leave the case with her winter clothes and documents in a locker at the airport for safekeeping and both had the key as he could have added a small bag of his own.
Did they then take the ferry from the Hook of Holland to England or a train from Heathrow and later hire a car?