You cannot use the companion voucher on JAL-operated flights. It can only be used on BA-operated flights.
If that is not an issue, JL 777 F is a very nice product (nowhere near as good IMO as the likes of SQ Suite but still very nice), and the food is substantially better than what you get on BA, but for Europe-TYO, it's very hard to get two seats on JL in F using points/miles if your dates are restricted, so realistically, if you have any date restrictions, you are quite likely to be looking at making a cash booking, for about £17,000 return for two persons for discounted F.
Last edited by LTN Phobia; Sep 16, 2018 at 12:43 pm