To the original question, I don't believe you can book an outbound then add the return. The voucher will be reflected in the charge for the oubound, then disappear. You probably could book a trip coming back the next day from TYO, i.e. Book a return the day after the seats first open. Then later shift the return date as desired (for £35).

If there are two of you with two vouchers, you could use one outbound and one inbound, on two bookings. Just make sure there's no meaningful YQ penalty.